Publications



2019

Journal Articles

Zacharias G. Datsios, Pantelis N. Mikropoulos and Thomas E. Tsovilis
IEEE Transactions on Electromagnetic Compatibility, 61, (3), pp. 623--630, 2019 Jan
[Abstract][BibTex][pdf]

@article{Datsios2019,
author={Zacharias G. Datsios and Pantelis N. Mikropoulos and Thomas E. Tsovilis},
title={Effects of Lightning Channel Equivalent Impedance on Lightning Performance of Overhead Transmission Lines},
journal={IEEE Transactions on Electromagnetic Compatibility},
volume={61},
number={3},
pages={623--630},
year={2019},
month={01},
date={2019-01-01},
url={https://doi.org/10.1109/temc.2019.2900420},
doi={http://10.1109/temc.2019.2900420}
}

2016

Journal Articles

Zacharias Datsios and Pantelis N. Mikropoulos
Electric Power Systems Research, 139, pp. 37-46, 2016 Jul
[Abstract][BibTex][pdf]

Evaluation of the dielectric strength of transmission line insulation subjected to fast-front overvoltages is of major importance for the insulation coordination of overhead lines and connected substations. Among models proposed in literature for the prediction of the dielectric behavior of long air gaps and insulators under lightning overvoltages, leader development models (also called leader progression models) have a greater physical significance. These models consider the predischarge current flowing in the gap during the leader propagation phase preceding breakdown. However, this current is often disregarded for simplicity in fast-front overvoltage simulations. In this study the effects of simulating predischarge current on voltage–time characteristics of long air gaps and insulators, critical currents of overhead transmission lines as well as fast-front overvoltages arising at substations are investigated with the aid of ATP–EMTP. Including predischarge current in simulations, affecting the flashover characteristics of air gaps and insulators, results in slightly higher estimates of the minimum backflashover current of overhead transmission lines. However, it does not affect the estimated minimum shielding failure flashover current of overhead lines. In addition, simulating predischarge current may affect the overvoltages arising at substations due to shielding failure of the connected overhead lines depending on withstand or flashover of line insulation. In the case of backflashover the wavefront steepness and amplitude of the overvoltages are lower. Thus, predischarge current effects should be considered in insulation coordination of overhead transmission lines and substations.

@article{2016Datsios,
author={Zacharias Datsios and Pantelis N. Mikropoulos},
title={Modeling of lightning impulse behavior of long air gaps and insulators including predischarge current: Implications on insulation coordination of overhead transmission lines and substations},
journal={Electric Power Systems Research},
volume={139},
pages={37-46},
year={2016},
month={07},
date={2016-07-01},
url={https://www.researchgate.net/publication/291165418_Modeling_of_lightning_impulse_behavior_of_long_air_gaps_and_insulators_including_predischarge_current_Implications_on_insulation_coordination_of_overhead_transmission_lines_and_substations},
doi={http://10.1016/j.epsr.2015.11.036},
abstract={Evaluation of the dielectric strength of transmission line insulation subjected to fast-front overvoltages is of major importance for the insulation coordination of overhead lines and connected substations. Among models proposed in literature for the prediction of the dielectric behavior of long air gaps and insulators under lightning overvoltages, leader development models (also called leader progression models) have a greater physical significance. These models consider the predischarge current flowing in the gap during the leader propagation phase preceding breakdown. However, this current is often disregarded for simplicity in fast-front overvoltage simulations. In this study the effects of simulating predischarge current on voltage–time characteristics of long air gaps and insulators, critical currents of overhead transmission lines as well as fast-front overvoltages arising at substations are investigated with the aid of ATP–EMTP. Including predischarge current in simulations, affecting the flashover characteristics of air gaps and insulators, results in slightly higher estimates of the minimum backflashover current of overhead transmission lines. However, it does not affect the estimated minimum shielding failure flashover current of overhead lines. In addition, simulating predischarge current may affect the overvoltages arising at substations due to shielding failure of the connected overhead lines depending on withstand or flashover of line insulation. In the case of backflashover the wavefront steepness and amplitude of the overvoltages are lower. Thus, predischarge current effects should be considered in insulation coordination of overhead transmission lines and substations.}
}

Pantelis N. Mikropoulos and Vasileios N. Zagkanas
IEEE Trans. on Dielectrics and Electrical Insulation, 23, (3), pp. 1322-1330, 2016 Aug
[Abstract][BibTex][pdf]

Characteristics of negative DC corona threshold inception in the coaxial cylindrical electrode arrangement are investigated based on experimental data and computations on avalanche growth considering a field and atmospheric conditions dependent photoelectron emission coefficient. A transition region between glow and streamer regimes of inception of the self-sustained corona discharge could be identified on the basis of space charge field effects on avalanche growth. Approximate expressions are introduced for satisfactorily estimating the negative corona inception field strength as interactively affected by conductor radius and atmospheric conditions. Polarity effects on corona inception characteristics are elucidated. The establishment of the glow corona generally occurs up to larger conductors for negative than positive corona; the opposite may also apply depending on atmospheric conditions. The inception field strength as well as its dependence on atmospheric conditions differs only slightly between negative and positive streamer corona; a critical avalanche number per unit length of ~108 cm-1 is required for both negative and positive streamer formation.

@article{Mikropoulos2016TDEI,
author={Pantelis N. Mikropoulos and Vasileios N. Zagkanas},
title={Negative DC corona inception in coaxial cylinders under variable atmospheric conditions: A comparison with positive corona},
journal={IEEE Trans. on Dielectrics and Electrical Insulation},
volume={23},
number={3},
pages={1322-1330},
year={2016},
month={08},
date={2016-08-05},
url={http://ieeexplore.ieee.org/document/7534609/},
doi={http://10.1109/TDEI.2015.005517},
issn={1070-9878},
abstract={Characteristics of negative DC corona threshold inception in the coaxial cylindrical electrode arrangement are investigated based on experimental data and computations on avalanche growth considering a field and atmospheric conditions dependent photoelectron emission coefficient. A transition region between glow and streamer regimes of inception of the self-sustained corona discharge could be identified on the basis of space charge field effects on avalanche growth. Approximate expressions are introduced for satisfactorily estimating the negative corona inception field strength as interactively affected by conductor radius and atmospheric conditions. Polarity effects on corona inception characteristics are elucidated. The establishment of the glow corona generally occurs up to larger conductors for negative than positive corona; the opposite may also apply depending on atmospheric conditions. The inception field strength as well as its dependence on atmospheric conditions differs only slightly between negative and positive streamer corona; a critical avalanche number per unit length of ~108 cm-1 is required for both negative and positive streamer formation.}
}

2015

Journal Articles

Pantelis N. Mikropoulos and Zacharias Datsios
IEEE Trans. on Dielectrics and Electrical Insulation, 22, (1), pp. 278-286, 2015 Feb
[Abstract][BibTex][pdf]

The conditions for threshold inception of positive DC corona in the coaxial cylindrical electrode arrangement are investigated. The establishment of the self-sustained corona discharge in glow or streamer form, as influenced by conductor radius and atmospheric conditions, is evaluated through detailed computations that consider space charge field effects on avalanche growth. The salient corona characteristics at threshold inception are assessed and discussed through comparisons with literature experimental data. In a wide range of inner conductor radius and atmospheric conditions corona initiates in streamer form when a critical avalanche number per unit length of about 108 cm-1 is attained. Accurate formulation of the critical field strength required to sustain ionization allowed for the derivation of approximate expressions accounting well for the interactive effects of conductor radius and atmospheric conditions on the corona inception field strength in both glow and streamer regimes.

@article{Mikropoulos2015TDEI,
author={Pantelis N. Mikropoulos and Zacharias Datsios},
title={Threshold inception conditions for positive DC corona in the coaxial cylindrical electrode arrangement under variable atmospheric conditions},
journal={IEEE Trans. on Dielectrics and Electrical Insulation},
volume={22},
number={1},
pages={278-286},
year={2015},
month={02},
date={2015-02-12},
url={http://ieeexplore.ieee.org/document/7033397/?arnumber=7033397&newsearch=true&queryText=Threshold%20inception%20conditions%20for%20positive%20DC%20corona%20in%20the%20coaxial%20cylindrical%20electrode%20arrangement%20under%20variable%20atmospheric%20conditions},
doi={http://10.1109/TDEI.2014.004641},
issn={1070-9878},
abstract={The conditions for threshold inception of positive DC corona in the coaxial cylindrical electrode arrangement are investigated. The establishment of the self-sustained corona discharge in glow or streamer form, as influenced by conductor radius and atmospheric conditions, is evaluated through detailed computations that consider space charge field effects on avalanche growth. The salient corona characteristics at threshold inception are assessed and discussed through comparisons with literature experimental data. In a wide range of inner conductor radius and atmospheric conditions corona initiates in streamer form when a critical avalanche number per unit length of about 108 cm-1 is attained. Accurate formulation of the critical field strength required to sustain ionization allowed for the derivation of approximate expressions accounting well for the interactive effects of conductor radius and atmospheric conditions on the corona inception field strength in both glow and streamer regimes.}
}

2015

Conference Papers

Zacharias Datsios and Pantelis N. Mikropoulos
19th International Symposium on High Voltage Engineering, Pilsen, Czech Republic, 2015 Aug
[Abstract][BibTex][pdf]

The evaluation of the lightning performance of overhead transmission lines necessitates the estimation of the minimum lightning current causing backflashover of line insulation. In the present study an assessment is made on the effects of transmission tower modelling upon the minimum backflashover current of typical 150 kV and 400 kV overhead lines with the aid of ATP-EMTP simulations. Several tower models, classified in three categories namely vertical lossless single-phase frequency-independent distributed parameter line models, multiconductor models and multistory models, were employed in simulations. Simulation results indicate that tower modelling, affecting the computed overvoltages arising across line insulators, influences the minimum backflashover current and the phase insulator associated with the latter. These effects are greatly dependent on tower ground resistance as well as upon the configuration of the transmission line.

@conference{Datsios2015,
author={Zacharias Datsios and Pantelis N. Mikropoulos},
title={Effect of tower modelling on the minimum backflashover current of overhead transmission lines},
booktitle={19th International Symposium on High Voltage Engineering},
address={Pilsen, Czech Republic},
year={2015},
month={08},
date={2015-08-01},
url={https://www.researchgate.net/publication/281345927_Effect_of_tower_modelling_on_the_minimum_backflashover_current_of_overhead_transmission_lines},
abstract={The evaluation of the lightning performance of overhead transmission lines necessitates the estimation of the minimum lightning current causing backflashover of line insulation. In the present study an assessment is made on the effects of transmission tower modelling upon the minimum backflashover current of typical 150 kV and 400 kV overhead lines with the aid of ATP-EMTP simulations. Several tower models, classified in three categories namely vertical lossless single-phase frequency-independent distributed parameter line models, multiconductor models and multistory models, were employed in simulations. Simulation results indicate that tower modelling, affecting the computed overvoltages arising across line insulators, influences the minimum backflashover current and the phase insulator associated with the latter. These effects are greatly dependent on tower ground resistance as well as upon the configuration of the transmission line.}
}

Nikolaos Mavrikakis, K. Siderakis, Pantelis N. Mikropoulos and N. Katsarakis
19th International Symposium on High Voltage Engineering, pp. 253, Pilsen, Czech Republic, 2015 Aug
[Abstract][BibTex][pdf]

Condition assessment results are presented for a field-aged 150 kV HTV SIR suspension insulator following its flashover due to bird nesting activity. The insulator, equipped with arcing horns, was operated for 10 years in a coastal transmission network under heavy environmental pollution. In order to evaluate flashover effects on insulator housing, several macroscopic and microscopic diagnostic techniques were employed including visual inspection, hydrophobicity classification, Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) analysis. Arcing affected the morphological structure and elemental composition, thus also hydrophobicity of the housing surface in proximity to arcing horns. The protection offered by the arcing horns together with the synthesis of the insulator housing material resulted in flashover deteriorating effects solely restricted to the upper surface layer of the housing. However, arcing caused excessive damages to the arcing horns. In addition to flashover, ageing and environmental pollution effects were detected on the insulator housing and further analysed.

@conference{Mavrikakis2015ISHVE,
author={Nikolaos Mavrikakis and K. Siderakis and Pantelis N. Mikropoulos and N. Katsarakis},
title={Condition assessment of a field-aged 150 kV HTV SIR suspension insulator following flashover},
booktitle={19th International Symposium on High Voltage Engineering},
pages={253},
address={Pilsen, Czech Republic},
year={2015},
month={08},
date={2015-08-01},
url={https://www.researchgate.net/publication/281370494_CONDITION_ASSESSMENT_OF_A_FIELD-AGED_150_kV_HTV_SIR_SUSPENSION_INSULATOR_FOLLOWING_FLASHOVER},
abstract={Condition assessment results are presented for a field-aged 150 kV HTV SIR suspension insulator following its flashover due to bird nesting activity. The insulator, equipped with arcing horns, was operated for 10 years in a coastal transmission network under heavy environmental pollution. In order to evaluate flashover effects on insulator housing, several macroscopic and microscopic diagnostic techniques were employed including visual inspection, hydrophobicity classification, Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) analysis. Arcing affected the morphological structure and elemental composition, thus also hydrophobicity of the housing surface in proximity to arcing horns. The protection offered by the arcing horns together with the synthesis of the insulator housing material resulted in flashover deteriorating effects solely restricted to the upper surface layer of the housing. However, arcing caused excessive damages to the arcing horns. In addition to flashover, ageing and environmental pollution effects were detected on the insulator housing and further analysed.}
}

Pantelis N. Mikropoulos, Thomas Tsovilis and V.S Victoratos
19th International Symposium on High Voltage Engineering, Pilsen, Czech Republic, 2015 Aug
[Abstract][BibTex][pdf]

Estimation of lightning incidence to vertical slender structures, such as telecommunication towers and wind turbines, is a formidable task. It is, however, of great importance in terms of reliability and insulation coordination of the connected power systems. The present study introduces a general methodology for the estimation of downward flashes to vertical slender structures on the basis of the statistical and several electrogeometric and generic lightning attachment models. The methodology has been incorporated in a user friendly mobile application, called TLIC. Lightning incidence calculations show that the relevant IEC 62305 procedure yields results deviating significantly from those obtained according to several lightning attachment models. It also, implementing an oversimplified geometrical approach, does not account for the dependence of lightning incidence on lightning peak current distribution and lightning interception probability distribution. With the aid of TLIC mobile application the effects of structure height, lightning peak current distribution and, based on the statistical model, lightning interception probability distribution are easily quantified. TLIC can be a useful tool for lightning risk assessment of vertical slender structures.

@conference{Mikropoulos2015ISHVE,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and V.S Victoratos},
title={Mobile application for the estimation of lightning incidence to vertical slender structures},
booktitle={19th International Symposium on High Voltage Engineering},
address={Pilsen, Czech Republic},
year={2015},
month={08},
date={2015-08-01},
url={https://www.researchgate.net/publication/281449283_MOBILE_APPLICATION_FOR_THE_ESTIMATION_OF_LIGHTNING_INCIDENCE_TO_VERTICAL_SLENDER_STRUCTURES},
abstract={Estimation of lightning incidence to vertical slender structures, such as telecommunication towers and wind turbines, is a formidable task. It is, however, of great importance in terms of reliability and insulation coordination of the connected power systems. The present study introduces a general methodology for the estimation of downward flashes to vertical slender structures on the basis of the statistical and several electrogeometric and generic lightning attachment models. The methodology has been incorporated in a user friendly mobile application, called TLIC. Lightning incidence calculations show that the relevant IEC 62305 procedure yields results deviating significantly from those obtained according to several lightning attachment models. It also, implementing an oversimplified geometrical approach, does not account for the dependence of lightning incidence on lightning peak current distribution and lightning interception probability distribution. With the aid of TLIC mobile application the effects of structure height, lightning peak current distribution and, based on the statistical model, lightning interception probability distribution are easily quantified. TLIC can be a useful tool for lightning risk assessment of vertical slender structures.}
}

2014

Journal Articles

Zacharias Datsios, Pantelis N. Mikropoulos and Thomas Tsovilis
Electric Power Systems Research, 113, pp. 141-150, 2014 Aug
[Abstract][BibTex][pdf]

ATP-EMTP simulations are performed to estimate the minimum shielding failure current causing flashover in overhead transmission lines with operating voltage in the range of 66 kV up to 735 kV. This critical current, affecting shielding failure flashover rate, is of great importance for assessing the insulation coordination of overhead transmission lines and the connected substations. The minimum shielding failure current causing flashover of line insulation is highly dependent upon insulator string flashover modelling and, also, markedly higher than that calculated according to the relevant IEEE Std 1243-1997 simplified expression. A modification of the latter is suggested by using multiplication factors of 1.5 and 1.65 for first and subsequent lightning strokes, respectively, so as to account for the increased dielectric strength of line insulator strings under non-standard lightning overvoltage surges. Alternatively, the critical currents can be respectively estimated by using average negative breakdown gradients per unit length of insulator string of 680 kV/m and 750 kV/m. The shielding failure flashover rate of the overhead transmission lines, being greatly influenced by insulator string flashover modelling, is lower than that obtained based on the critical current according to IEEE Std 1243-1997.

@article{Datsios2014,
author={Zacharias Datsios and Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Estimation of the minimum shielding failure flashover current for first and subsequent lightning strokes to overhead transmission lines},
journal={Electric Power Systems Research},
volume={113},
pages={141-150},
year={2014},
month={08},
date={2014-08-01},
url={https://www.sciencedirect.com/science/article/pii/S0378779614000959},
doi={http://doi.org/10.1016/j.epsr.2014.03.008},
issn={0378-7796},
abstract={ATP-EMTP simulations are performed to estimate the minimum shielding failure current causing flashover in overhead transmission lines with operating voltage in the range of 66 kV up to 735 kV. This critical current, affecting shielding failure flashover rate, is of great importance for assessing the insulation coordination of overhead transmission lines and the connected substations. The minimum shielding failure current causing flashover of line insulation is highly dependent upon insulator string flashover modelling and, also, markedly higher than that calculated according to the relevant IEEE Std 1243-1997 simplified expression. A modification of the latter is suggested by using multiplication factors of 1.5 and 1.65 for first and subsequent lightning strokes, respectively, so as to account for the increased dielectric strength of line insulator strings under non-standard lightning overvoltage surges. Alternatively, the critical currents can be respectively estimated by using average negative breakdown gradients per unit length of insulator string of 680 kV/m and 750 kV/m. The shielding failure flashover rate of the overhead transmission lines, being greatly influenced by insulator string flashover modelling, is lower than that obtained based on the critical current according to IEEE Std 1243-1997.}
}

Pantelis N. Mikropoulos, Thomas Tsovilis and Sotiria G. Koutoula
IEEE Trans. on Power Delivery, 29, (6), pp. 2570-2579, 2014 Jun
[Abstract][BibTex][pdf]

Data on unscheduled electric service interruptions in an extended distribution network (20/0.4 kV) were analyzed in order to examine the causes contributing to distribution substation failures. A distribution substation feeding a Global System for Mobile Communications base station experienced the highest service interruption rate due to transformer sustained failures, the vast majority of which were categorized as lightning related. Thus, with the aid of detailed Alternate Transients Program-Electromagnetic Transients Program simulations, an investigation on the overvoltages surges impinging on the distribution transformer due to direct lightning strokes to the connected MV overhead line and to the nearby telecommunication tower has been made. Transformer failures are caused solely by fast-front overvoltages exceeding the basic insulation level of the LV side of the transformer. This is substantiated by the good agreement between estimated and reported transformer failure rates. The safe operation of the distribution transformer necessitates the installation of surge protective devices at its low-voltage terminals, also overcoming the need for extremely low values of telecommunication tower grounding resistance. Additional line surge arresters should be installed at the penultimate wood pole of the connected overhead line which, besides improving the lightning performance of the transformer, significantly increases the reliability of the distribution substation.

@article{Mikropoulos2014,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and Sotiria G. Koutoula},
title={Lightning performance of distribution transformer feeding GSM base station},
journal={IEEE Trans. on Power Delivery},
volume={29},
number={6},
pages={2570-2579},
year={2014},
month={06},
date={2014-06-22},
url={http://ieeexplore.ieee.org/document/6862079/?tp=&arnumber=6862079},
doi={http://10.1109/TPWRD.2014.2335253},
issn={0885-8977},
abstract={Data on unscheduled electric service interruptions in an extended distribution network (20/0.4 kV) were analyzed in order to examine the causes contributing to distribution substation failures. A distribution substation feeding a Global System for Mobile Communications base station experienced the highest service interruption rate due to transformer sustained failures, the vast majority of which were categorized as lightning related. Thus, with the aid of detailed Alternate Transients Program-Electromagnetic Transients Program simulations, an investigation on the overvoltages surges impinging on the distribution transformer due to direct lightning strokes to the connected MV overhead line and to the nearby telecommunication tower has been made. Transformer failures are caused solely by fast-front overvoltages exceeding the basic insulation level of the LV side of the transformer. This is substantiated by the good agreement between estimated and reported transformer failure rates. The safe operation of the distribution transformer necessitates the installation of surge protective devices at its low-voltage terminals, also overcoming the need for extremely low values of telecommunication tower grounding resistance. Additional line surge arresters should be installed at the penultimate wood pole of the connected overhead line which, besides improving the lightning performance of the transformer, significantly increases the reliability of the distribution substation.}
}

2014

Conference Papers

Zacharias Datsios and Pantelis N. Mikropoulos
2014 International Conference on Lightning Protection (ICLP), pp. 979-985, 2014 Nov
[Abstract][BibTex][pdf]

Evaluation of the dielectric strength of transmission line insulation subjected to fast-front overvoltages is of major importance for the insulation coordination of overhead lines and the connected substations. Among models proposed in literature for the prediction of the dielectric behavior of air gaps and insulators, leader development models have a greater physical significance. In this study several leader development models are implemented in a new model in ATP-EMTP using MODELS simulation language. The new model is solved simultaneously with the rest of simulated system interacting with it as a circuit element; this is accomplished by using the model with a type-94 circuit component. The developed model is verified against literature data referring to breakdown of long air gaps. Shielding failure and backflashover simulations are conducted for a 150 kV and a 400 kV overhead transmission line to demonstrate the effects of modeling of the predischarge current flowing during leader propagation phase on the estimated critical currents.

@conference{6973265,
author={Zacharias Datsios and Pantelis N. Mikropoulos},
title={Implementation of leader development models in ATP-EMTP Using a type-94 circuit component},
booktitle={2014 International Conference on Lightning Protection (ICLP)},
pages={979-985},
year={2014},
month={11},
date={2014-11-01},
url={http://ieeexplore.ieee.org/abstract/document/6973265/},
doi={http://10.1109/ICLP.2014.6973265},
abstract={Evaluation of the dielectric strength of transmission line insulation subjected to fast-front overvoltages is of major importance for the insulation coordination of overhead lines and the connected substations. Among models proposed in literature for the prediction of the dielectric behavior of air gaps and insulators, leader development models have a greater physical significance. In this study several leader development models are implemented in a new model in ATP-EMTP using MODELS simulation language. The new model is solved simultaneously with the rest of simulated system interacting with it as a circuit element; this is accomplished by using the model with a type-94 circuit component. The developed model is verified against literature data referring to breakdown of long air gaps. Shielding failure and backflashover simulations are conducted for a 150 kV and a 400 kV overhead transmission line to demonstrate the effects of modeling of the predischarge current flowing during leader propagation phase on the estimated critical currents.}
}

Zacharias Datsios, Pantelis N. Mikropoulos, Andreas Teneketzoglou and Dimitrios Tzikas
49th Universities Power Engineering Conference, pp. 1-6, IEEE, Cluj-Napoca, Romania, 2014 Sep
[Abstract][BibTex][pdf]

The design of the grounding configuration for the metal fence of a high voltage installation is important as the outside perimeter of the fence is accessible to the general public. In this work the safety provided by several fence grounding techniques commonly used in high voltage installations is evaluated for a 150/20 kV air insulated substation, a 400 kV step-up GIS substation and a large scale photovoltaic power station with the aid of grounding analysis software. A safe and cost-efficient fence grounding design depends on ground fault characteristics, soil conditions, installation area, distance between the fence and grounding grid as well as on the size and geometry of the latter.

@conference{Datsios2014UPEC,
author={Zacharias Datsios and Pantelis N. Mikropoulos and Andreas Teneketzoglou and Dimitrios Tzikas},
title={Safety performance evaluation of fence grounding configurations in high voltage installations},
booktitle={49th Universities Power Engineering Conference},
pages={1-6},
publisher={IEEE},
address={Cluj-Napoca, Romania},
year={2014},
month={09},
date={2014-09-02},
url={http://ieeexplore.ieee.org/document/6934650/?arnumber=6934650},
doi={http://10.1109/UPEC.2014.6934650},
abstract={The design of the grounding configuration for the metal fence of a high voltage installation is important as the outside perimeter of the fence is accessible to the general public. In this work the safety provided by several fence grounding techniques commonly used in high voltage installations is evaluated for a 150/20 kV air insulated substation, a 400 kV step-up GIS substation and a large scale photovoltaic power station with the aid of grounding analysis software. A safe and cost-efficient fence grounding design depends on ground fault characteristics, soil conditions, installation area, distance between the fence and grounding grid as well as on the size and geometry of the latter.}
}

Nikolaos Mavrikakis, K. Siderakis and Pantelis N. Mikropoulos
49th Universities Power Engineering Conference, pp. 1-6, IEEE, Cluj-Napoca, Romania, 2014 Sep
[Abstract][BibTex][pdf]

Evaluation of the field performance of composite insulators is essential for maintaining the desired levels of reliability in power networks. Field performance of composite insulators depends on many factors, including the insulator design, material quality and service conditions experienced. The most critical part of the composite insulator is its external housing, usually made of silicone rubber having the advantage of recovering its hydrophobic properties in polluted conditions. Thus, condition assessment of the hydrophobic properties of the housing material especially under erosion and tracking processes is of major importance. In this study performance evaluation of the housing material of 150 kV field-aged silicone rubber insulators is carried out through physical, electrical and material analysis techniques. The hydrophobic properties of the housing material were found degraded to a different extent between field-aged insulators due to differences in material structure and pollution conditions. Hydrophobicity can be better assessed if the relevant diagnostic techniques are also applied after inclined plane tests.

@conference{Mavrikakis2014UPEC,
author={Nikolaos Mavrikakis and K. Siderakis and Pantelis N. Mikropoulos},
title={Laboratory investigation on hydrophobicity and tracking performance of field aged composite insulators},
booktitle={49th Universities Power Engineering Conference},
pages={1-6},
publisher={IEEE},
address={Cluj-Napoca, Romania},
year={2014},
month={09},
date={2014-09-02},
url={http://ieeexplore.ieee.org/document/6934665/?tp=&arnumber=6934665},
doi={http://%2010.1109/UPEC.2014.6934665},
abstract={Evaluation of the field performance of composite insulators is essential for maintaining the desired levels of reliability in power networks. Field performance of composite insulators depends on many factors, including the insulator design, material quality and service conditions experienced. The most critical part of the composite insulator is its external housing, usually made of silicone rubber having the advantage of recovering its hydrophobic properties in polluted conditions. Thus, condition assessment of the hydrophobic properties of the housing material especially under erosion and tracking processes is of major importance. In this study performance evaluation of the housing material of 150 kV field-aged silicone rubber insulators is carried out through physical, electrical and material analysis techniques. The hydrophobic properties of the housing material were found degraded to a different extent between field-aged insulators due to differences in material structure and pollution conditions. Hydrophobicity can be better assessed if the relevant diagnostic techniques are also applied after inclined plane tests.}
}

Pantelis N. Mikropoulos, Thomas Tsovilis and Aikaterini S. Pori
32nd International Conference on Lightning Protection, pp. 1212-1216, Shanghai, China, 2014 Oct
[Abstract][BibTex][pdf]

The lightning performance of overhead distributions lines affects significantly the reliability of distribution power network. Lightning-related flashover of line insulation is caused by direct and nearby strokes. This paper presents a generalized methodology for the estimation of the flashover rate of overhead distribution lines that considers several lightning attachment and coupling models. Results of the proposed methodology are discussed and compared with A. J. Eriksson\'s field observations. Based on the consistency between estimated values and field data suggestions are made for the appropriate selection of lightning attachment and coupling models for the evaluation of the lightning performance of distribution lines.

@conference{Mikropoulos2014ICLP,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and Aikaterini S. Pori},
title={Evaluation of lightning attachment and coupling models for the estimation of the lightning performance of overhead distribution lines},
booktitle={32nd International Conference on Lightning Protection},
pages={1212-1216},
address={Shanghai, China},
year={2014},
month={10},
date={2014-10-11},
url={http://ieeexplore.ieee.org/document/6973314/},
doi={http://10.1109/ICLP.2014.6973314},
abstract={The lightning performance of overhead distributions lines affects significantly the reliability of distribution power network. Lightning-related flashover of line insulation is caused by direct and nearby strokes. This paper presents a generalized methodology for the estimation of the flashover rate of overhead distribution lines that considers several lightning attachment and coupling models. Results of the proposed methodology are discussed and compared with A. J. Eriksson\\'s field observations. Based on the consistency between estimated values and field data suggestions are made for the appropriate selection of lightning attachment and coupling models for the evaluation of the lightning performance of distribution lines.}
}

2013

Journal Articles

Pantelis N. Mikropoulos and Thomas Tsovilis
IEEE Trans. on Dielectrics and Electrical Insulation, 20, (1), pp. 202-211, 2013 Feb
[Abstract][BibTex][pdf]

Lightning is a major cause of distribution line outages affecting reliability of power supply thus, consequently, resulting in economic losses. The lightning performance of overhead distribution lines is associated with line insulation flashover due to direct and nearby lightning strokes. The present paper introduces a statistical method for the estimation of the flashover rate of overhead distribution lines. The proposed method, implementing a recently introduced statistical lightning attachment model and the simplified coupling model suggested by the IEEE Std. 1410:2011, yields a range for the expected flashover rate of distribution lines associated with lightning interception probability distribution of the line conductors. It also provides the means to easily quantify the effects of line parameters, soil resistivity, shielding of nearby objects and lightning crest current distribution on lightning performance of distribution lines. Results of the statistical method, discussed and compared with that obtained by the IEEE Std. 1410:2011, are in close consistency with field data.

@article{Mikropoulos2013TDEI,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Statistical method for the evaluation of the lightning performance of overhead distribution lines},
journal={IEEE Trans. on Dielectrics and Electrical Insulation},
volume={20},
number={1},
pages={202-211},
year={2013},
month={02},
date={2013-02-11},
url={http://ieeexplore.ieee.org/document/6451359/?tp=&arnumber=6451359},
doi={http://10.1109/TDEI.2013.6451359},
issn={1070-9878},
abstract={Lightning is a major cause of distribution line outages affecting reliability of power supply thus, consequently, resulting in economic losses. The lightning performance of overhead distribution lines is associated with line insulation flashover due to direct and nearby lightning strokes. The present paper introduces a statistical method for the estimation of the flashover rate of overhead distribution lines. The proposed method, implementing a recently introduced statistical lightning attachment model and the simplified coupling model suggested by the IEEE Std. 1410:2011, yields a range for the expected flashover rate of distribution lines associated with lightning interception probability distribution of the line conductors. It also provides the means to easily quantify the effects of line parameters, soil resistivity, shielding of nearby objects and lightning crest current distribution on lightning performance of distribution lines. Results of the statistical method, discussed and compared with that obtained by the IEEE Std. 1410:2011, are in close consistency with field data.}
}

2013

Conference Papers

Zacharias Datsios, Pantelis N. Mikropoulos, Z. Politis, A.G Kagiannas and Thomas Tsovilis
18th International Symposium on High Voltage Engineering, Seoul, Korea, 2013 Aug
[Abstract][BibTex][pdf]

The effects of the length of the surge arrester connection conductors on the lightning surges impinging on a typical wood pole-mounted 50 kVA, 20/0.4 kV transformer of the Hellenic distribution system are investigated through detailed ATP-EMTP simulations. By considering both first and subsequent direct lightning strokes to a connected overhead distribution line, the effectiveness of the common practice transformer protection scheme and of an alternative one utilizing shorter surge arrester connection conductors in suppressing fast-front overvoltages was evaluated. A shorter length of the surge arrester connection conductors results in a reduction in the amplitude of the overvoltages arising at the medium-voltage terminals of the transformer and in a slower rate of increase of the overvoltage amplitude with lightning return-stroke current. The overvoltages transferred to the low-voltage terminals of the transformer are practically not affected by the length of the surge arrester connection conductors. Protection against transferred overvoltages was provided by surge protective devices installed at the low-voltage terminals of the transformer. By utilizing shorter surge arrester connection conductors the transformer failure rate, estimated through risk assessment, is reduced by approximately 11%.

@conference{Datsios2013ISHVE,
author={Zacharias Datsios and Pantelis N. Mikropoulos and Z. Politis and A.G Kagiannas and Thomas Tsovilis},
title={Protection of distribution transformer against arising or transferred fast-front overvoltages: Effects of surge arrester connection conductors length},
booktitle={18th International Symposium on High Voltage Engineering},
address={Seoul, Korea},
year={2013},
month={08},
date={2013-08-01},
url={https://www.researchgate.net/publication/256292931_Protection_of_distribution_transformer_against_arising_or_transferred_fast-front_overvoltages_Effects_of_surge_arrester_connection_conductors_length},
abstract={The effects of the length of the surge arrester connection conductors on the lightning surges impinging on a typical wood pole-mounted 50 kVA, 20/0.4 kV transformer of the Hellenic distribution system are investigated through detailed ATP-EMTP simulations. By considering both first and subsequent direct lightning strokes to a connected overhead distribution line, the effectiveness of the common practice transformer protection scheme and of an alternative one utilizing shorter surge arrester connection conductors in suppressing fast-front overvoltages was evaluated. A shorter length of the surge arrester connection conductors results in a reduction in the amplitude of the overvoltages arising at the medium-voltage terminals of the transformer and in a slower rate of increase of the overvoltage amplitude with lightning return-stroke current. The overvoltages transferred to the low-voltage terminals of the transformer are practically not affected by the length of the surge arrester connection conductors. Protection against transferred overvoltages was provided by surge protective devices installed at the low-voltage terminals of the transformer. By utilizing shorter surge arrester connection conductors the transformer failure rate, estimated through risk assessment, is reduced by approximately 11%.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
18th International Symposium on High Voltage Engineering, Seoul, Korea, 2013 Aug
[Abstract][BibTex][pdf]

A methodology for the estimation of lightning incidence to free standing Early Streamer Emission (ESE) rods is introduced. The proposed method implements a general expression for the equivalent interception radius, which considers the ESE rod height, length of the triggered upward connecting leader and the lightning peak current distribution. Lightning incidence results are discussed and compared with that referring to conventional rods obtained according to Eriksson’s method and the statistical lightning attachment model. It is shown that the lightning interception efficiency of an ESE rod, even assuming an earlier initiated upward connecting leader, is similar to that of a conventional rod when considering a realistic value for the progression velocity of the upward connecting leader. The present work provides the means to easily evaluate the lightning interception efficiency of ESE rods through comparisons with available field data.

@conference{Mikropoulos2013ISHVE,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Evaluation of lightning incidence to ESE rods},
booktitle={18th International Symposium on High Voltage Engineering},
address={Seoul, Korea},
year={2013},
month={08},
date={2013-08-01},
url={https://www.researchgate.net/publication/256353092_Evaluation_of_lightning_incidence_to_ESE_rods},
abstract={A methodology for the estimation of lightning incidence to free standing Early Streamer Emission (ESE) rods is introduced. The proposed method implements a general expression for the equivalent interception radius, which considers the ESE rod height, length of the triggered upward connecting leader and the lightning peak current distribution. Lightning incidence results are discussed and compared with that referring to conventional rods obtained according to Eriksson’s method and the statistical lightning attachment model. It is shown that the lightning interception efficiency of an ESE rod, even assuming an earlier initiated upward connecting leader, is similar to that of a conventional rod when considering a realistic value for the progression velocity of the upward connecting leader. The present work provides the means to easily evaluate the lightning interception efficiency of ESE rods through comparisons with available field data.}
}

Pantelis N. Mikropoulos and Vasileios Zagkanas
48th Universities Power Engineering Conference, pp. 1-5, Dublin, Ireland, 2013 Sep
[Abstract][BibTex][pdf]

The effect of stranded conductor geometry on the salient characteristics of DC corona, namely corona inception voltage, corona conductance and the associated power losses is experimentally investigated in the coaxial cylindrical electrode arrangement in air. The number of strands of the conductor has a greater impact on the characteristics of positive than negative corona. Under conditions of high absolute humidity, corona conductance, thus also losses depending on conductor geometry can be higher for positive than negative corona. Breakdown, only slightly affected by stranded conductor geometry, is greatly facilitated by the space charge associated with multiple coronas occurring along the energised conductor. An empirical expression for the estimation of the irregularity factor of overhead transmission line conductors is proposed, accounting well for the effects of stranded conductor radius, stranding ratio and relative air density.

@conference{mikropoulos2013UPEC,
author={Pantelis N. Mikropoulos and Vasileios Zagkanas},
title={The effect of stranded conductor geometry on DC corona in the coaxial cylindrical electrode arrangement in air},
booktitle={48th Universities Power Engineering Conference},
pages={1-5},
address={Dublin, Ireland},
year={2013},
month={09},
date={2013-09-02},
url={http://ieeexplore.ieee.org/document/6714987/?tp=&arnumber=6714987},
doi={http://10.1109/UPEC.2013.6714987},
abstract={The effect of stranded conductor geometry on the salient characteristics of DC corona, namely corona inception voltage, corona conductance and the associated power losses is experimentally investigated in the coaxial cylindrical electrode arrangement in air. The number of strands of the conductor has a greater impact on the characteristics of positive than negative corona. Under conditions of high absolute humidity, corona conductance, thus also losses depending on conductor geometry can be higher for positive than negative corona. Breakdown, only slightly affected by stranded conductor geometry, is greatly facilitated by the space charge associated with multiple coronas occurring along the energised conductor. An empirical expression for the estimation of the irregularity factor of overhead transmission line conductors is proposed, accounting well for the effects of stranded conductor radius, stranding ratio and relative air density.}
}

Pantelis N. Mikropoulos and Vasileios Zagkanas
18th International Symposium on High Voltage Engineering, Seoul, Korea, 2013 Aug
[Abstract][BibTex][pdf]

The effect of the steepness of overvoltage surges propagating along overhead transmission lines on corona characteristics is of great importance for insulation coordination studies. Corona inception voltage on line conductors under fast-front overvoltages is commonly estimated through Peek’s empirical formula, which however was obtained for steady or slow-varying electric fields. In the present study experimental results on impulse corona in the coaxial cylindrical electrode arrangement in air are presented, with the wavefront duration and the polarity of the applied voltage as parameters. Corona discharge has been investigated at both threshold and overstress conditions by obtaining inception probability distributions and U-t curves. An empirical expression describes well the experimentally derived U-t characteristic. A modified Peek’s formula is proposed taking into account the effect of the steepness of the applied impulse voltage on corona inception field strength. The integration method, commonly used for the estimation of the impulse breakdown voltage of non-uniform air gaps, has been implemented to estimate the corona inception time and voltage; a very good agreement exists between calculated and measured values.

@conference{Mikropoulos2013Zagkanas,
author={Pantelis N. Mikropoulos and Vasileios Zagkanas},
title={Impulse corona inception in the coaxial cylindrical electrode arrangement in air: effects of the steepness of the applied voltage},
booktitle={18th International Symposium on High Voltage Engineering},
address={Seoul, Korea},
year={2013},
month={08},
date={2013-08-01},
url={https://www.researchgate.net/publication/256741973_Impulse_corona_inception_in_the_coaxial_cylindrical_electrode_arrangement_in_air_effects_of_the_steepness_of_the_applied_voltage},
abstract={The effect of the steepness of overvoltage surges propagating along overhead transmission lines on corona characteristics is of great importance for insulation coordination studies. Corona inception voltage on line conductors under fast-front overvoltages is commonly estimated through Peek’s empirical formula, which however was obtained for steady or slow-varying electric fields. In the present study experimental results on impulse corona in the coaxial cylindrical electrode arrangement in air are presented, with the wavefront duration and the polarity of the applied voltage as parameters. Corona discharge has been investigated at both threshold and overstress conditions by obtaining inception probability distributions and U-t curves. An empirical expression describes well the experimentally derived U-t characteristic. A modified Peek’s formula is proposed taking into account the effect of the steepness of the applied impulse voltage on corona inception field strength. The integration method, commonly used for the estimation of the impulse breakdown voltage of non-uniform air gaps, has been implemented to estimate the corona inception time and voltage; a very good agreement exists between calculated and measured values.}
}

2012

Journal Articles

Petros Mavroidis, Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
IEEE Trans. on Dielectrics and Electrical Insulation, 19, (2), pp. 632-640, 2012 Apr
[Abstract][BibTex][pdf]

The impulse behavior of composite short rod-plane air gaps with a dielectric-covered rod under positive lightning and switching impulse voltages is investigated, by monitoring the electric field strength at the earthed plane, by still photographs of corona discharges and through electric field computations. Experiments were performed in normal laboratory air at pressure around 0.1 MPa, temperatures in the range 19 to 28°C and absolute and relative humidity varied naturally between 11-21 g/m3 and 60-90%, respectively. Breakdown probability distributions were obtained for composite gaps with a dielectric cover made of either PTFE or epoxy resin and for simple air gaps with a bare rod. Also, basic characteristic parameters of coronas preceding breakdown in composite gaps, namely initial corona in the vicinity of the covering tip, surface coronas emerging along the dielectric cover and developing mainly in air, and secondary corona in the vicinity of the cover upper end, were measured. Surface corona development is greatly affected by initial corona growth and facilitates secondary corona inception. Breakdown, occurring at higher voltages for composite gaps, is closely related with the secondary corona inception; a similar dependence upon impulse waveshape and cover material was seen to exist. At breakdown, the spark channel bridges the gap that includes the dielectric cover length. The spark channel develops either along the cover surface or partly or solely in free air, being affected by surface corona development.

@article{Mavroidis2012TDEI,
author={Petros Mavroidis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Impulse behavior of dielectric-covered rod-plane air gaps},
journal={IEEE Trans. on Dielectrics and Electrical Insulation},
volume={19},
number={2},
pages={632-640},
year={2012},
month={04},
date={2012-04-16},
url={http://ieeexplore.ieee.org/document/6180258/?tp=&arnumber=6180258},
doi={http://10.1109/TDEI.2012.6180258},
issn={1070-9878},
abstract={The impulse behavior of composite short rod-plane air gaps with a dielectric-covered rod under positive lightning and switching impulse voltages is investigated, by monitoring the electric field strength at the earthed plane, by still photographs of corona discharges and through electric field computations. Experiments were performed in normal laboratory air at pressure around 0.1 MPa, temperatures in the range 19 to 28°C and absolute and relative humidity varied naturally between 11-21 g/m3 and 60-90%, respectively. Breakdown probability distributions were obtained for composite gaps with a dielectric cover made of either PTFE or epoxy resin and for simple air gaps with a bare rod. Also, basic characteristic parameters of coronas preceding breakdown in composite gaps, namely initial corona in the vicinity of the covering tip, surface coronas emerging along the dielectric cover and developing mainly in air, and secondary corona in the vicinity of the cover upper end, were measured. Surface corona development is greatly affected by initial corona growth and facilitates secondary corona inception. Breakdown, occurring at higher voltages for composite gaps, is closely related with the secondary corona inception; a similar dependence upon impulse waveshape and cover material was seen to exist. At breakdown, the spark channel bridges the gap that includes the dielectric cover length. The spark channel develops either along the cover surface or partly or solely in free air, being affected by surface corona development.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
IEEE Trans. on Dielectrics and Electrical Insulation, 19, (6), pp. 2155-2164, 2012 Dec
[Abstract][BibTex][pdf]

Shielding failure of overhead transmission lines is a major cause of transmission system outages, affecting reliability of power supply and resulting in economic losses. In this study the shielding performance of overhead transmission lines is evaluated with the aid of general expressions, derived by implementing in shielding analysis different categories of lightning attachment models. Thus, the effects of lightning attachment model, transmission line parameters and lightning crest current distribution on shielding failure rate of overhead transmission lines have been quantified. Alternative approaches to that proposed by IEEE Std 1243 for assessing the shielding performance of transmission lines, including computer simulations of lightning attachment, are evaluated. Shielding failure rate results are discussed and compared with field data reported in literature. For typical overhead transmission lines, an upper limit of the estimated shielding failure rate is 0.4% of the rate of lightning strokes to the line. More and reliable field data is needed in order to evaluate lightning attachment models with respect to the lightning performance of overhead transmission lines.

@article{Mikropoulos2012TDEI,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Estimation of the shielding performance of overhead transmission lines: The effects of lightning attachment model and lightning crest current distribution},
journal={IEEE Trans. on Dielectrics and Electrical Insulation},
volume={19},
number={6},
pages={2155-2164},
year={2012},
month={12},
date={2012-12-01},
url={http://ieeexplore.ieee.org/document/6396976/?tp=&arnumber=6396976},
doi={http://10.1109/TDEI.2012.6396976},
issn={1070-9878},
abstract={Shielding failure of overhead transmission lines is a major cause of transmission system outages, affecting reliability of power supply and resulting in economic losses. In this study the shielding performance of overhead transmission lines is evaluated with the aid of general expressions, derived by implementing in shielding analysis different categories of lightning attachment models. Thus, the effects of lightning attachment model, transmission line parameters and lightning crest current distribution on shielding failure rate of overhead transmission lines have been quantified. Alternative approaches to that proposed by IEEE Std 1243 for assessing the shielding performance of transmission lines, including computer simulations of lightning attachment, are evaluated. Shielding failure rate results are discussed and compared with field data reported in literature. For typical overhead transmission lines, an upper limit of the estimated shielding failure rate is 0.4% of the rate of lightning strokes to the line. More and reliable field data is needed in order to evaluate lightning attachment models with respect to the lightning performance of overhead transmission lines.}
}

2012

Conference Papers

Zacharias Datsios, Pantelis N. Mikropoulos and Thomas Tsovilis
31st International Conference on Lightning Protection, pp. 1-5, Vienna, Austria, 2012 Sep
[Abstract][BibTex][pdf]

ATP-EMTP simulations are performed to estimate the minimum shielding failure current causing flashover in 150 kV and 400 kV double-circuit overhead lines of the Hellenic transmission system. This critical current is of great importance for estimating the shielding failure flashover rate and assessing the insulation coordination of overhead transmission lines. It is shown that the minimum shielding failure current causing flashover varies significantly among the insulator string flashover models employed in simulations, which include volt-time curves and leader development models. The minimum shielding failure current causing flashover calculated on the basis of the relevant IEEE Std is significantly lower than that obtained through ATP-EMTP simulations. The computed shielding failure flashover rate of the 150 kV and 400 kV overhead transmission lines is greatly affected by the method adopted for the estimation of the minimum shielding failure current causing flashover.

@conference{datsios2012,
author={Zacharias Datsios and Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Estimation of the minimum shielding failure current causing flashover in overhead lines of the Hellenic transmission system},
booktitle={31st International Conference on Lightning Protection},
pages={1-5},
address={Vienna, Austria},
year={2012},
month={09},
date={2012-09-02},
url={http://ieeexplore.ieee.org/document/6344378/?tp=&arnumber=6344378},
doi={http://10.1109/ICLP.2012.6344378},
abstract={ATP-EMTP simulations are performed to estimate the minimum shielding failure current causing flashover in 150 kV and 400 kV double-circuit overhead lines of the Hellenic transmission system. This critical current is of great importance for estimating the shielding failure flashover rate and assessing the insulation coordination of overhead transmission lines. It is shown that the minimum shielding failure current causing flashover varies significantly among the insulator string flashover models employed in simulations, which include volt-time curves and leader development models. The minimum shielding failure current causing flashover calculated on the basis of the relevant IEEE Std is significantly lower than that obtained through ATP-EMTP simulations. The computed shielding failure flashover rate of the 150 kV and 400 kV overhead transmission lines is greatly affected by the method adopted for the estimation of the minimum shielding failure current causing flashover.}
}

Zacharias Datsios and Pantelis N. Mikropoulos
MedPower 2012, pp. 1-6, Cagliari, Italy, 2012 Oct
[Abstract][BibTex][pdf]

A safe and cost-efficient grounding system design of a 3 MWp photovoltaic power station according to IEEE Std 80-2000 is presented. Grounding analysis is performed by considering the metal parts of the photovoltaic panel arrays foundations as auxiliary ground electrodes. Utilizing also horizontal ground conductors, required solely for the interconnections of the metal support structures of the photovoltaic panel arrays, both safety and cost-efficiency in grounding system design have been achieved. It is shown that in large-scale photovoltaic power stations where the metal parts of the panel arrays foundations are concrete encased the concrete resistivity is not an important parameter in evaluating the safety performance of the grounding system.

@conference{datsios2012MP,
author={Zacharias Datsios and Pantelis N. Mikropoulos},
title={Safe grounding system design for a photovoltaic power station},
booktitle={MedPower 2012},
pages={1-6},
address={Cagliari, Italy},
year={2012},
month={10},
date={2012-10-01},
url={http://ieeexplore.ieee.org/document/6521882/?tp=&arnumber=6521882},
doi={http://10.1049/cp.2012.2041},
abstract={A safe and cost-efficient grounding system design of a 3 MWp photovoltaic power station according to IEEE Std 80-2000 is presented. Grounding analysis is performed by considering the metal parts of the photovoltaic panel arrays foundations as auxiliary ground electrodes. Utilizing also horizontal ground conductors, required solely for the interconnections of the metal support structures of the photovoltaic panel arrays, both safety and cost-efficiency in grounding system design have been achieved. It is shown that in large-scale photovoltaic power stations where the metal parts of the panel arrays foundations are concrete encased the concrete resistivity is not an important parameter in evaluating the safety performance of the grounding system.}
}

Zacharias Datsios, Pantelis N. Mikropoulos and Thomas Tsovilis
46th Universities Power Engineering Conference, pp. 1-5, VDE, Soest, Germany, 2012 Sep
[Abstract][BibTex][pdf]

Accurate modeling of line insulation flashover is very important in evaluating the fast-front surges arising at overhead transmission lines and impinging on the connected substations. A new ATPDraw object, called ISF, has been developed with the aid of which insulator string flashover is modeled on the basis of several flashover models from literature. The ISF object was employed in ATP-EMTP simulations of a 150 kV GIS substation. The computed backflashover surges impinging on the substation vary considerably among the insulator string flashover models employed in simulations. The ISF object is a useful tool within the ATP-EMTP environment for insulation co-ordination studies. The effects of insulator string flashover modeling on backflashover surges arising at overhead transmission lines and impinging on the connected substations can be easily quantified.

@conference{Datsios2012UPEC,
author={Zacharias Datsios and Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Insulator string flashover modelling with the aid of an ATPDraw object},
booktitle={46th Universities Power Engineering Conference},
pages={1-5},
publisher={VDE},
address={Soest, Germany},
year={2012},
month={09},
date={2012-09-05},
url={http://ieeexplore.ieee.org/document/6125515/},
abstract={Accurate modeling of line insulation flashover is very important in evaluating the fast-front surges arising at overhead transmission lines and impinging on the connected substations. A new ATPDraw object, called ISF, has been developed with the aid of which insulator string flashover is modeled on the basis of several flashover models from literature. The ISF object was employed in ATP-EMTP simulations of a 150 kV GIS substation. The computed backflashover surges impinging on the substation vary considerably among the insulator string flashover models employed in simulations. The ISF object is a useful tool within the ATP-EMTP environment for insulation co-ordination studies. The effects of insulator string flashover modeling on backflashover surges arising at overhead transmission lines and impinging on the connected substations can be easily quantified.}
}

Pantelis N. Mikropoulos, Thomas Tsovilis and P.P Papaioannou
2012 47th International Universities Power Engineering Conference (UPEC), pp. 1-5, London, UK, 2012 Sep
[Abstract][BibTex][pdf]

A useful application software for the evaluation of the lightning performance of overhead distribution lines is introduced. The SM-LPDL software has been developed in Matlab, runs as a Microsoft Windows application and features a user-friendly graphics interface. It incorporates a statistical method for the estimation of the flashover rate of overhead distribution lines, implementing a recently introduced statistical lightning attachment model and the simplified coupling model suggested by the IEEE Std. 1410:2011. The SM-LPDL yields a range for the expected flashover rate of a distribution line associated with the lightning interception probability distribution of the line conductors. It allows for the easy quantification of the effects of line parameters, soil resistivity, surge arresters, shielding of nearby objects and lightning crest current distribution on lightning performance of distribution lines. SM-LPDL results are discussed and compared with those obtained by the recently released IEEE Std. 1410:2011.

@conference{Mikropoulos2012,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and P.P Papaioannou},
title={Software development for the evaluation of the lightning performance of overhead distribution lines},
booktitle={2012 47th International Universities Power Engineering Conference (UPEC)},
pages={1-5},
address={London, UK},
year={2012},
month={09},
date={2012-09-04},
url={http://ieeexplore.ieee.org/document/6398442/?reload=true},
doi={http://10.1109/UPEC.2012.6398442},
abstract={A useful application software for the evaluation of the lightning performance of overhead distribution lines is introduced. The SM-LPDL software has been developed in Matlab, runs as a Microsoft Windows application and features a user-friendly graphics interface. It incorporates a statistical method for the estimation of the flashover rate of overhead distribution lines, implementing a recently introduced statistical lightning attachment model and the simplified coupling model suggested by the IEEE Std. 1410:2011. The SM-LPDL yields a range for the expected flashover rate of a distribution line associated with the lightning interception probability distribution of the line conductors. It allows for the easy quantification of the effects of line parameters, soil resistivity, surge arresters, shielding of nearby objects and lightning crest current distribution on lightning performance of distribution lines. SM-LPDL results are discussed and compared with those obtained by the recently released IEEE Std. 1410:2011.}
}

Pantelis N. Mikropoulos, Thomas Tsovilis and P.P Papaioannou
47th Universities Power Engineering Conference, pp. 1-5, London, United Kingdom, 2012 Sep
[Abstract][BibTex][pdf]

A useful application software for the evaluation of the lightning performance of overhead distribution lines is introduced. The SM-LPDL software has been developed in Matlab, runs as a Microsoft Windows application and features a user-friendly graphics interface. It incorporates a statistical method for the estimation of the flashover rate of overhead distribution lines, implementing a recently introduced statistical lightning attachment model and the simplified coupling model suggested by the IEEE Std. 1410:2011. The SM-LPDL yields a range for the expected flashover rate of a distribution line associated with the lightning interception probability distribution of the line conductors. It allows for the easy quantification of the effects of line parameters, soil resistivity, surge arresters, shielding of nearby objects and lightning crest current distribution on lightning performance of distribution lines. SM-LPDL results are discussed and compared with those obtained by the recently released IEEE Std. 1410:2011.

@conference{Mikropoulos2012UPEC,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and P.P Papaioannou},
title={Software development for the evaluation of the lightning performance of overhead distribution lines on the basis of the statistical method},
booktitle={47th Universities Power Engineering Conference},
pages={1-5},
address={London, United Kingdom},
year={2012},
month={09},
date={2012-09-04},
url={http://ieeexplore.ieee.org/document/6398442/?tp=&arnumber=6398442},
doi={http://10.1109/UPEC.2012.6398442},
abstract={A useful application software for the evaluation of the lightning performance of overhead distribution lines is introduced. The SM-LPDL software has been developed in Matlab, runs as a Microsoft Windows application and features a user-friendly graphics interface. It incorporates a statistical method for the estimation of the flashover rate of overhead distribution lines, implementing a recently introduced statistical lightning attachment model and the simplified coupling model suggested by the IEEE Std. 1410:2011. The SM-LPDL yields a range for the expected flashover rate of a distribution line associated with the lightning interception probability distribution of the line conductors. It allows for the easy quantification of the effects of line parameters, soil resistivity, surge arresters, shielding of nearby objects and lightning crest current distribution on lightning performance of distribution lines. SM-LPDL results are discussed and compared with those obtained by the recently released IEEE Std. 1410:2011.}
}

Pantelis N. Mikropoulos, Vasileios Zagkanas and T.S. Koustoulidis
47th Universities Power Engineering Conference, pp. 1-6, London, United Kingdom, 2012 Sep
[Abstract][BibTex][pdf]

Knowledge on the effects of air density on corona discharge is of great importance for many practical applications, including overhead transmission lines. In the present study the salient characteristics of both positive and negative DC corona on 7-stranded conductors, namely the corona inception voltage, corona current and the associated power losses, are experimentally investigated in the coaxial cylindrical electrode arrangement under variable air density. Theoretical predictions of the corona current and losses are in good agreement with experimental values for an ion mobility decreasing with relative air density according to a power law, the rate of decrease being higher for negative than positive ions. Irregularity factors for the 7-stranded conductors have been obtained based on electric field simulations and Peek\'s formula for smooth conductors. Logarithmic expressions, describing well the dependence of the irregularity factor on the product of relative air density and conductor radius, allow for a satisfactory estimation of the corona inception field strength of 7-stranded conductors.

@conference{Mikropoulos2012Zagkanas,
author={Pantelis N. Mikropoulos and Vasileios Zagkanas and T.S. Koustoulidis},
title={Experimental investigation of DC corona on stranded conductors under variable air density},
booktitle={47th Universities Power Engineering Conference},
pages={1-6},
address={London, United Kingdom},
year={2012},
month={09},
date={2012-09-04},
url={http://ieeexplore.ieee.org/document/6398421/},
doi={http://10.1109/UPEC.2012.6398421},
abstract={Knowledge on the effects of air density on corona discharge is of great importance for many practical applications, including overhead transmission lines. In the present study the salient characteristics of both positive and negative DC corona on 7-stranded conductors, namely the corona inception voltage, corona current and the associated power losses, are experimentally investigated in the coaxial cylindrical electrode arrangement under variable air density. Theoretical predictions of the corona current and losses are in good agreement with experimental values for an ion mobility decreasing with relative air density according to a power law, the rate of decrease being higher for negative than positive ions. Irregularity factors for the 7-stranded conductors have been obtained based on electric field simulations and Peek\\'s formula for smooth conductors. Logarithmic expressions, describing well the dependence of the irregularity factor on the product of relative air density and conductor radius, allow for a satisfactory estimation of the corona inception field strength of 7-stranded conductors.}
}

2011

Journal Articles

Lazaros Lazaridis and Pantelis N. Mikropoulos
IEEE Trans. on Dielectrics and Electrical Insulation, 18, (3), pp. 745-754, 2011 Jul
[Abstract][BibTex][pdf]

Positive impulse flashover along smooth cylindrical insulating surfaces bridging a short rod-plane gap under standard lightning and switching impulse voltages is investigated under variable absolute humidity and by regarding the case of air alone as reference. Flashover is closely related to predischarge phenomena, specifically secondary corona under lightning impulses and corona streamers arrival at the earthed plane under switching impulse voltages. Under lightning impulses the dielectric strength of the gap is highest for air alone and decreases with material permittivity; the opposite is true, although less marked, under switching impulse voltages. Flashover voltage increases with absolute humidity, this being more pronounced for switching than lightning impulse voltages. The inhibitory effect of humidity on flashover, mainly determined by the trajectory of the spark channel with respect to the insulating surface, is similar to that observed in air alone where flashover occurs away from the insulating surface but reduced where the discharge develops close to the insulating surface. For insulating arrangements where at flashover the spark channel adheres to the insulating surface the IEC 60060-1:1989 atmospheric conditions correction procedure should be optimized by considering a reduced humidity correction factor.

@article{Lazaridis2011,
author={Lazaros Lazaridis and Pantelis N. Mikropoulos},
title={Positive impulse flashover along smooth cylindrical insulating surfaces under variable humidity},
journal={IEEE Trans. on Dielectrics and Electrical Insulation},
volume={18},
number={3},
pages={745-754},
year={2011},
month={07},
date={2011-07-30},
url={http://ieeexplore.ieee.org/document/5931061/?arnumber=5931061&tag=1},
doi={http://10.1109/TDEI.2011.5931061},
issn={1070-9878},
abstract={Positive impulse flashover along smooth cylindrical insulating surfaces bridging a short rod-plane gap under standard lightning and switching impulse voltages is investigated under variable absolute humidity and by regarding the case of air alone as reference. Flashover is closely related to predischarge phenomena, specifically secondary corona under lightning impulses and corona streamers arrival at the earthed plane under switching impulse voltages. Under lightning impulses the dielectric strength of the gap is highest for air alone and decreases with material permittivity; the opposite is true, although less marked, under switching impulse voltages. Flashover voltage increases with absolute humidity, this being more pronounced for switching than lightning impulse voltages. The inhibitory effect of humidity on flashover, mainly determined by the trajectory of the spark channel with respect to the insulating surface, is similar to that observed in air alone where flashover occurs away from the insulating surface but reduced where the discharge develops close to the insulating surface. For insulating arrangements where at flashover the spark channel adheres to the insulating surface the IEC 60060-1:1989 atmospheric conditions correction procedure should be optimized by considering a reduced humidity correction factor.}
}

2011

Conference Papers

Zacharias Datsios, Pantelis N. Mikropoulos and Thomas Tsovilis
International Conference on Power Systems Transients, Delft, The Netherlands, 2011 Jun
[Abstract][BibTex][pdf]

Tower grounding system accurate modeling is very important in evaluating the backflashover surges arising at overhead transmission lines and impinging on the connected substations. A new ATPDraw object, called TGIR, has been developed with the aid of which a concentrated tower grounding system can be represented on the basis of several tower grounding system models. The TGIR object was employed in ATP-EMTP simulations of a 150 kV GIS substation. The computed backflashover surges impinging on the substation vary considerably among the tower grounding system models employed in simulations, as a result of the variability in the grounding impulse resistance. The TGIR object is a useful tool within the ATP-EMTP environment for insulation co-ordination studies; the effects of tower grounding system modeling on backflashover surges arising at overhead transmission lines and impinging on the connected substations can be easily quantified.

@conference{Datsios2011ICPST,
author={Zacharias Datsios and Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Impulse resistance of concentrated tower grounding systems simulated by an ATPDraw object},
booktitle={International Conference on Power Systems Transients},
address={Delft, The Netherlands},
year={2011},
month={06},
date={2011-06-01},
url={https://www.researchgate.net/publication/230817376_Impulse_resistance_of_concentrated_tower_grounding_systems_simulated_by_an_ATPDraw_object},
abstract={Tower grounding system accurate modeling is very important in evaluating the backflashover surges arising at overhead transmission lines and impinging on the connected substations. A new ATPDraw object, called TGIR, has been developed with the aid of which a concentrated tower grounding system can be represented on the basis of several tower grounding system models. The TGIR object was employed in ATP-EMTP simulations of a 150 kV GIS substation. The computed backflashover surges impinging on the substation vary considerably among the tower grounding system models employed in simulations, as a result of the variability in the grounding impulse resistance. The TGIR object is a useful tool within the ATP-EMTP environment for insulation co-ordination studies; the effects of tower grounding system modeling on backflashover surges arising at overhead transmission lines and impinging on the connected substations can be easily quantified.}
}

Zacharias Datsios, Pantelis N. Mikropoulos and Thomas Tsovilis
International Conference on Power Systems Transients, 2011 Jun
[Abstract][BibTex][pdf]

The maximum shielding failure current of overhead transmission lines is an important parameter in evaluating the shielding performance of the lines and performing insulation coordination studies of the connected substations. A new ATPDraw object has been developed, by using MODELS language, that easily generates the maximum shielding failure current of overhead transmission lines, with amplitude and waveshape depending on line geometry and selected lightning attachment model. The new object, called MSFC, was employed in ATP-EMTP simulations of a 150 kV GIS substation. The computed shielding failure surges impinging on the substation, being dependent upon maximum shielding failure current, vary considerably among lightning attachment models. The MSFC object is a useful tool within the ATP-EMTP environment in assessing the shielding failure surges arising at overhead transmission lines and impinging on the connected substations. Shielding failure current of overhead transmission lines generated through an ATPDraw object (PDF Download Available). Available from: https://www.researchgate.net/publication/230817373_Shielding_failure_current_of_overhead_transmission_lines_generated_through_an_ATPDraw_object [accessed Feb 05 2018].

@conference{Datsios2011ICPSTMikropoulos,
author={Zacharias Datsios and Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Shielding failure current of overhead transmissions lines generated through an ATPDraw object},
booktitle={International Conference on Power Systems Transients},
year={2011},
month={06},
date={2011-06-01},
url={https://www.researchgate.net/publication/230817373_Shielding_failure_current_of_overhead_transmission_lines_generated_through_an_ATPDraw_object},
abstract={The maximum shielding failure current of overhead transmission lines is an important parameter in evaluating the shielding performance of the lines and performing insulation coordination studies of the connected substations. A new ATPDraw object has been developed, by using MODELS language, that easily generates the maximum shielding failure current of overhead transmission lines, with amplitude and waveshape depending on line geometry and selected lightning attachment model. The new object, called MSFC, was employed in ATP-EMTP simulations of a 150 kV GIS substation. The computed shielding failure surges impinging on the substation, being dependent upon maximum shielding failure current, vary considerably among lightning attachment models. The MSFC object is a useful tool within the ATP-EMTP environment in assessing the shielding failure surges arising at overhead transmission lines and impinging on the connected substations. Shielding failure current of overhead transmission lines generated through an ATPDraw object (PDF Download Available). Available from: https://www.researchgate.net/publication/230817373_Shielding_failure_current_of_overhead_transmission_lines_generated_through_an_ATPDraw_object [accessed Feb 05 2018].}
}

Pantelis N. Mikropoulos and Vasileios Zagkanas
17th International Symposium on High Voltage Engineering, Hannover, Germany, 2011 Aug
[Abstract][BibTex][pdf]

Corona discharge has attracted much interest among researchers as it has a lot of applications in industry and a number of effects on power systems. Corona inception in the coaxial cylindrical electrode arrangement, which finds many practical applications, depends on the electric field strength around the inner conductor and on atmospheric conditions. In the present study a new model for positive corona inception in a steady or slowly-varying electric field is presented. The model, implementing streamer criterion, assumes that an equivalent electron avalanche develops towards the anode by virtue of ionization by collision and photoionization in an electric field distorted by the avalanche space charge. Based on a great amount of literature experimental data, a new empirical expression for the estimation of positive corona inception field strength has been derived. The formulation of a photoionization coefficient as a function of inner conductor radius and relative air density enabled the investigation of the effects of the latter parameters on the basic characteristics of the avalanche at corona inception, namely critical avalanche length, radius, number and electron density.

@conference{Mikropoulos2011ISHVE,
author={Pantelis N. Mikropoulos and Vasileios Zagkanas},
title={Modelling of positive corona inception in the coaxial cylindrical electrode arrangement under variable air density},
booktitle={17th International Symposium on High Voltage Engineering},
address={Hannover, Germany},
year={2011},
month={08},
date={2011-08-01},
url={https://www.researchgate.net/publication/235799564_Modelling_of_positive_corona_inception_in_the_coaxial_cylindrical_electrode_arrangement_under_variable_air_density},
abstract={Corona discharge has attracted much interest among researchers as it has a lot of applications in industry and a number of effects on power systems. Corona inception in the coaxial cylindrical electrode arrangement, which finds many practical applications, depends on the electric field strength around the inner conductor and on atmospheric conditions. In the present study a new model for positive corona inception in a steady or slowly-varying electric field is presented. The model, implementing streamer criterion, assumes that an equivalent electron avalanche develops towards the anode by virtue of ionization by collision and photoionization in an electric field distorted by the avalanche space charge. Based on a great amount of literature experimental data, a new empirical expression for the estimation of positive corona inception field strength has been derived. The formulation of a photoionization coefficient as a function of inner conductor radius and relative air density enabled the investigation of the effects of the latter parameters on the basic characteristics of the avalanche at corona inception, namely critical avalanche length, radius, number and electron density.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
17th International Symposium on High Voltage Engineering, Hannover, Germany, 2011 Aug
[Abstract][BibTex][pdf]

Lightning is a major cause of distribution line outages affecting reliability of power supply thus, consequently, resulting in economic losses. Line insulation flashover in overhead distribution lines may be caused by overvoltages associated with direct or nearby lightning strokes. In this paper a statistical method for the estimation of the induced-voltage flashover rate of unshielded overhead distribution lines due to nearby strokes is introduced. The proposed method yields a range for the expected inducedvoltage flashover rate of the line, by considering, besides line parameters and lightning crest current distribution, the lightning interception probability distribution of the line phase conductors. Results of the statistical method are compared with those yielded by the methods suggested in IEEE Std. 1410:2004 and its recent revision IEEE Std. 1410:2011; a satisfactory agreement is shown to exist. An application of the statistical method to a typical 20 kV unshielded line of the Hellenic distribution system is made. A statistical method for the estimation of induced-voltage flashover rate of unshielded overhead distribution lines (PDF Download Available). Available from: https://www.researchgate.net/publication/235986109_A_statistical_method_for_the_estimation_of_induced-voltage_flashover_rate_of_unshielded_overhead_distribution_lines [accessed Feb 05 2018].

@conference{Mikropoulos2011Tsovilis,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={A statistical method for the estimation of induced-voltage flashover rate of unshielded overhead distribution lines},
booktitle={17th International Symposium on High Voltage Engineering},
address={Hannover, Germany},
year={2011},
month={08},
date={2011-08-01},
url={https://www.researchgate.net/publication/235986109_A_statistical_method_for_the_estimation_of_induced-voltage_flashover_rate_of_unshielded_overhead_distribution_lines},
abstract={Lightning is a major cause of distribution line outages affecting reliability of power supply thus, consequently, resulting in economic losses. Line insulation flashover in overhead distribution lines may be caused by overvoltages associated with direct or nearby lightning strokes. In this paper a statistical method for the estimation of the induced-voltage flashover rate of unshielded overhead distribution lines due to nearby strokes is introduced. The proposed method yields a range for the expected inducedvoltage flashover rate of the line, by considering, besides line parameters and lightning crest current distribution, the lightning interception probability distribution of the line phase conductors. Results of the statistical method are compared with those yielded by the methods suggested in IEEE Std. 1410:2004 and its recent revision IEEE Std. 1410:2011; a satisfactory agreement is shown to exist. An application of the statistical method to a typical 20 kV unshielded line of the Hellenic distribution system is made. A statistical method for the estimation of induced-voltage flashover rate of unshielded overhead distribution lines (PDF Download Available). Available from: https://www.researchgate.net/publication/235986109_A_statistical_method_for_the_estimation_of_induced-voltage_flashover_rate_of_unshielded_overhead_distribution_lines [accessed Feb 05 2018].}
}

2010

Journal Articles

Lazaros Lazaridis and Pantelis N. Mikropoulos
IEEE Trans. on Dielectrics and Electrical Insulation, 17, (5), pp. 1585-1591, 2010 Oct
[Abstract][BibTex][pdf]

Negative impulse flashover along insulating surfaces bridging a short rod-plane gap is investigated under variable humidity. The specimens, cylindrical in shape and with a smooth surface, were made of PTFE, silicone-rubber, nylon and glazed porcelain and were bridging the gap, which was stressed by standard lightning and switching impulse voltages. Breakdown probability distributions were obtained and the breakdown voltage and time to breakdown were measured. The gap was overstressed by applying voltages higher than that causing 100% breakdown so as to study the effects of the applied peak voltage on breakdown characteristics. When breakdown occurs over an insulating surface the breakdown voltage is significantly lower compared to that obtained for air alone, especially at lower breakdown probabilities and under switching impulse voltages. It is also lower for lightning than switching impulse voltages, this being less marked with increasing material permittivity. Breakdown is closely related to negative corona growth. The effect of absolute humidity on breakdown voltage is minimal. The IEC atmospheric conditions correction procedure yields satisfactory results when breakdown occurs both in air alone and along an insulating surface.

@article{Lazaridis2010,
author={Lazaros Lazaridis and Pantelis N. Mikropoulos},
title={Negative impulse flashover along cylindrical insulating surfaces bridging a short rod-plane gap under variable humidity},
journal={IEEE Trans. on Dielectrics and Electrical Insulation},
volume={17},
number={5},
pages={1585-1591},
year={2010},
month={10},
date={2010-10-07},
url={http://ieeexplore.ieee.org/document/5595561/?arnumber=5595561&tag=1},
doi={http://10.1109/TDEI.2010.5595561},
issn={1070-9878},
abstract={Negative impulse flashover along insulating surfaces bridging a short rod-plane gap is investigated under variable humidity. The specimens, cylindrical in shape and with a smooth surface, were made of PTFE, silicone-rubber, nylon and glazed porcelain and were bridging the gap, which was stressed by standard lightning and switching impulse voltages. Breakdown probability distributions were obtained and the breakdown voltage and time to breakdown were measured. The gap was overstressed by applying voltages higher than that causing 100% breakdown so as to study the effects of the applied peak voltage on breakdown characteristics. When breakdown occurs over an insulating surface the breakdown voltage is significantly lower compared to that obtained for air alone, especially at lower breakdown probabilities and under switching impulse voltages. It is also lower for lightning than switching impulse voltages, this being less marked with increasing material permittivity. Breakdown is closely related to negative corona growth. The effect of absolute humidity on breakdown voltage is minimal. The IEC atmospheric conditions correction procedure yields satisfactory results when breakdown occurs both in air alone and along an insulating surface.}
}

Lazaros Lazaridis and Pantelis N. Mikropoulos
"Positive lightning impulse discharges along cylindrical insulating surfaces bridging a short rod-plane gap"
IET Sci. Meas. Technol, 4, (2), pp. 71-83, 2010 Mar
[Abstract][BibTex]

The salient characteristics of the predischarge phenomena and flashover along cylindrical insulating surfaces bridging a short rod-plane gap under positive lightning impulse voltages were measured in a wide range of applied peak voltages. Materials used in the investigation were polytetrafluoroethylene (PTFE), silicone-rubber, nylon and glazed porcelain; the case of air alone was regarded as reference. Corona inception is favoured by the presence of an insulating surface, this being more evident for materials of high permittivity and for initial than secondary corona. At relatively high applied voltages, the initial corona characteristics are directly related to the field enhancement at the tip of the stressed electrode. However, near threshold inception conditions there is a marked reduction of the inception field with material permittivity; an assisting mechanism of initiatory electron production was put forward to explain this behaviour. Initial corona streamers require higher applied voltages to arrive at the earthed plane when propagating along an insulating surface than in air alone. Flashover, closely related to secondary corona characteristics, is always preceded by continuous corona growth, occurring mainly in free air away from the insulating surface because of possible positive surface charge deposition by the initial corona; the latter is characteristic for polymeric insulating surfaces. There is a reduction of the dielectric strength of the gap in the presence of an insulating surface, more pronounced for higher material permittivity, related to the facilitating action of the field enhancement close to the rod tip on continuous corona growth preceding flashover.

@article{Lazaridis2010IET,
author={Lazaros Lazaridis and Pantelis N. Mikropoulos},
title={Positive lightning impulse discharges along cylindrical insulating surfaces bridging a short rod-plane gap},
journal={IET Sci. Meas. Technol},
volume={4},
number={2},
pages={71-83},
year={2010},
month={03},
date={2010-03-01},
doi={http://10.1049/iet-smt.2009.0082},
issn={1751-8822},
abstract={The salient characteristics of the predischarge phenomena and flashover along cylindrical insulating surfaces bridging a short rod-plane gap under positive lightning impulse voltages were measured in a wide range of applied peak voltages. Materials used in the investigation were polytetrafluoroethylene (PTFE), silicone-rubber, nylon and glazed porcelain; the case of air alone was regarded as reference. Corona inception is favoured by the presence of an insulating surface, this being more evident for materials of high permittivity and for initial than secondary corona. At relatively high applied voltages, the initial corona characteristics are directly related to the field enhancement at the tip of the stressed electrode. However, near threshold inception conditions there is a marked reduction of the inception field with material permittivity; an assisting mechanism of initiatory electron production was put forward to explain this behaviour. Initial corona streamers require higher applied voltages to arrive at the earthed plane when propagating along an insulating surface than in air alone. Flashover, closely related to secondary corona characteristics, is always preceded by continuous corona growth, occurring mainly in free air away from the insulating surface because of possible positive surface charge deposition by the initial corona; the latter is characteristic for polymeric insulating surfaces. There is a reduction of the dielectric strength of the gap in the presence of an insulating surface, more pronounced for higher material permittivity, related to the facilitating action of the field enhancement close to the rod tip on continuous corona growth preceding flashover.}
}

Petros Mavroidis, Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
IET Sci. Meas. Technol, 4, (2), pp. 52-61, 2010 Mar
[Abstract][BibTex][pdf]

The breakdown mechanism of dielectric–covered rod/plane air gaps under positive lightning impulse voltages is investigated. Several gap lengths, varying between 2.5 and 15?cm, were employed and the simple air gaps were regarded as reference. The basic characteristics of coronas, namely inception time and voltage, were measured at applied voltages just sufficient for their inception up to voltages causing breakdown. Multiple level tests were conducted; thus, corona inception and breakdown probability distributions were obtained and time to breakdown was measured at several voltage levels. Interpretation of the results was made based on oscillograms of the electric field strength at the earthed plane, which was monitored through a capacitive probe, geometric field calculations and still photographs of the discharge at breakdown. In the dielectric-covered rod/plane gaps, breakdown occurs at higher applied voltages as a result of increase in both corona inception voltages and discharge path. The development of the discharge depends on the electric field distribution as modified by both space and surface charges associated with coronas preceding breakdown. Surface charge accumulation may affect the discharge path at breakdown.

@article{Mavroidis2010,
author={Petros Mavroidis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Lightning impulse behaviour of short rod-plane gaps with a dielectric-covered rod},
journal={IET Sci. Meas. Technol},
volume={4},
number={2},
pages={52-61},
year={2010},
month={03},
date={2010-03-01},
url={http://digital-library.theiet.org/content/journals/10.1049/iet-smt.2008.0137},
doi={http://10.1049/iet-smt.2008.0137},
issn={1751-8822},
abstract={The breakdown mechanism of dielectric–covered rod/plane air gaps under positive lightning impulse voltages is investigated. Several gap lengths, varying between 2.5 and 15?cm, were employed and the simple air gaps were regarded as reference. The basic characteristics of coronas, namely inception time and voltage, were measured at applied voltages just sufficient for their inception up to voltages causing breakdown. Multiple level tests were conducted; thus, corona inception and breakdown probability distributions were obtained and time to breakdown was measured at several voltage levels. Interpretation of the results was made based on oscillograms of the electric field strength at the earthed plane, which was monitored through a capacitive probe, geometric field calculations and still photographs of the discharge at breakdown. In the dielectric-covered rod/plane gaps, breakdown occurs at higher applied voltages as a result of increase in both corona inception voltages and discharge path. The development of the discharge depends on the electric field distribution as modified by both space and surface charges associated with coronas preceding breakdown. Surface charge accumulation may affect the discharge path at breakdown.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
IET Generation, Transmission and Distribution, 4, (12), pp. 1299-1313, 2010 Dec
[Abstract][BibTex][pdf]

The maximum shielding failure current of overhead transmission lines is an important parameter in evaluating the shielding performance of the lines and in insulation coordination of substations. General expressions for the estimation of the maximum shielding failure current of transmission lines, derived by employing several lightning attachment models in shielding analysis, are presented. An application to typical 110?kV up to 1150?kV overhead transmission lines shows that there is a great variability in maximum shielding failure current among lightning attachment models. The importance of maximum shielding failure current in insulation coordination of substations is demonstrated with the aid of alternative transients program-electromagnetic transients program (ATP-EMTP) simulations. The computed overvoltages impinging on 150 and 400?kV gas insulated system (GIS) substations because of shielding failure of the incoming overhead transmission lines, being dependent upon shielding failure current, vary with the lightning attachment model employed in shielding analysis of the lines. Implementation of the electrogeometric model adopted by IEEE Std 1243:1997 in shielding analysis imposes high requirements on protection of the substations against incoming shielding failure surges.

@article{Mikropoulos2010,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Lightning attachment models and maximum shielding failure current of overhead transmission lines: Implications in insulation coordination of substations},
journal={IET Generation, Transmission and Distribution},
volume={4},
number={12},
pages={1299-1313},
year={2010},
month={12},
date={2010-12-01},
url={http://digital-library.theiet.org/content/journals/10.1049/iet-gtd.2009.0685},
doi={http://10.1049/iet-gtd.2009.0685%20},
abstract={The maximum shielding failure current of overhead transmission lines is an important parameter in evaluating the shielding performance of the lines and in insulation coordination of substations. General expressions for the estimation of the maximum shielding failure current of transmission lines, derived by employing several lightning attachment models in shielding analysis, are presented. An application to typical 110?kV up to 1150?kV overhead transmission lines shows that there is a great variability in maximum shielding failure current among lightning attachment models. The importance of maximum shielding failure current in insulation coordination of substations is demonstrated with the aid of alternative transients program-electromagnetic transients program (ATP-EMTP) simulations. The computed overvoltages impinging on 150 and 400?kV gas insulated system (GIS) substations because of shielding failure of the incoming overhead transmission lines, being dependent upon shielding failure current, vary with the lightning attachment model employed in shielding analysis of the lines. Implementation of the electrogeometric model adopted by IEEE Std 1243:1997 in shielding analysis imposes high requirements on protection of the substations against incoming shielding failure surges.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
"Interception probability and proximity effects: Implications in shielding design against lightning"
IEEE Trans. on Power Delivery, 25, (3), pp. 1940-1951, 2010 Mar
[Abstract][BibTex]

The effects of a neighboring object on the interception probability distribution of an air terminal are investigated through scale model experiments. The discharge interception probability of an earthed rod is influenced by the presence of a neighboring shorter one when both inserted in a rod-plane gap. This depends on the polarity of the impulse voltage, the height of the earthed rods and their displacement with respect to the energized rod. The experimental results are interpreted in terms of the breakdown mechanism and electric field calculations; their possible extension to shielding design is discussed. Proximity effects may result in a reduction in the striking distance and interception radius of an air terminal, especially at the lower interception probabilities. A new approach for shielding design is introduced, which considers, besides lightning parameters and height of the prospective struck objects, interception probability and proximity effects.

@article{Mikropoulos2010TPD,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Interception probability and proximity effects: Implications in shielding design against lightning},
journal={IEEE Trans. on Power Delivery},
volume={25},
number={3},
pages={1940-1951},
year={2010},
month={03},
date={2010-03-29},
doi={http://%2010.1109/TPWRD.2010.2043692},
abstract={The effects of a neighboring object on the interception probability distribution of an air terminal are investigated through scale model experiments. The discharge interception probability of an earthed rod is influenced by the presence of a neighboring shorter one when both inserted in a rod-plane gap. This depends on the polarity of the impulse voltage, the height of the earthed rods and their displacement with respect to the energized rod. The experimental results are interpreted in terms of the breakdown mechanism and electric field calculations; their possible extension to shielding design is discussed. Proximity effects may result in a reduction in the striking distance and interception radius of an air terminal, especially at the lower interception probabilities. A new approach for shielding design is introduced, which considers, besides lightning parameters and height of the prospective struck objects, interception probability and proximity effects.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
"Estimation of lightning incidence to overhead transmission lines"
IEEE Trans. on Power Delivery, 25, (3), pp. 1855-1865, 2010 Jun
[Abstract][BibTex]

General expressions for the estimation of lightning incidence to overhead transmission lines on the basis of electrogeometric and generic models are introduced, which consider, besides transmission-line geometry, lightning crest current distribution and, based on the recently proposed statistical model, interception probability distribution. An application to typical 115 kV up to 765 kV and large scale 500 kV and UHV overhead transmission lines is performed and the computed results, varying significantly among lightning attachment models, are validated through comparisons with field data from literature; the IEEE Std 1243 overestimates significantly lightning incidence to shield wires of large scale transmission lines. The expected annual number of lightning strikes to shield wires of transmission lines depends on lightning crest current distribution; this dependence, easily quantified with the aid of the introduced general expressions, is not considered by the relevant IEEE standard. Lightning incidence results are discussed in the context of the backflashover rate of overhead transmission lines.

@article{Mikropoulos2010TPD06,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Estimation of lightning incidence to overhead transmission lines},
journal={IEEE Trans. on Power Delivery},
volume={25},
number={3},
pages={1855-1865},
year={2010},
month={06},
date={2010-06-01},
doi={http://10.1109/TPWRD.2010.2046918},
abstract={General expressions for the estimation of lightning incidence to overhead transmission lines on the basis of electrogeometric and generic models are introduced, which consider, besides transmission-line geometry, lightning crest current distribution and, based on the recently proposed statistical model, interception probability distribution. An application to typical 115 kV up to 765 kV and large scale 500 kV and UHV overhead transmission lines is performed and the computed results, varying significantly among lightning attachment models, are validated through comparisons with field data from literature; the IEEE Std 1243 overestimates significantly lightning incidence to shield wires of large scale transmission lines. The expected annual number of lightning strikes to shield wires of transmission lines depends on lightning crest current distribution; this dependence, easily quantified with the aid of the introduced general expressions, is not considered by the relevant IEEE standard. Lightning incidence results are discussed in the context of the backflashover rate of overhead transmission lines.}
}

2010

Conference Papers

Pantelis N. Mikropoulos, Thomas Tsovilis, Zacharias Datsios and Nikolaos Mavrikakis
45th Universities Power Engineering Conference, pp. 1-6, Cardiff, Wales, 2010 Aug
[Abstract][BibTex][pdf]

Overvoltages arising in 150 kV and 400 kV GIS substations due to backflashover of the incoming overhead transmission lines were computed with the aid of ATP-EMTP simulations, by considering the effects of several simulation models of the basic transmission line components. The protection offered against impinging surges by surge arresters operating at the substation entrance is evaluated with respect to the basic insulation level of the GIS system. The computed overvoltages vary considerably among tower grounding system models and among insulator string flashover models whereas rather insignificantly among tower simulation models. There is no systematic variation in computed overvoltages among insulator string flashover models. Single vertical lossless line models and a constant rather than a current dependent resistance are considered, in terms of simulation simplicity and safe design, as satisfactory for simulating transmission line tower and its grounding resistance, respectively, in insulation coordination studies of substations.

@conference{5649203,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and Zacharias Datsios and Nikolaos Mavrikakis},
title={Effects of simulation models of overhead transmission line basic components on backflashover surges impinging on GIS substations},
booktitle={45th Universities Power Engineering Conference},
pages={1-6},
address={Cardiff, Wales},
year={2010},
month={08},
date={2010-08-31},
url={http://ieeexplore.ieee.org/document/5649203/},
abstract={Overvoltages arising in 150 kV and 400 kV GIS substations due to backflashover of the incoming overhead transmission lines were computed with the aid of ATP-EMTP simulations, by considering the effects of several simulation models of the basic transmission line components. The protection offered against impinging surges by surge arresters operating at the substation entrance is evaluated with respect to the basic insulation level of the GIS system. The computed overvoltages vary considerably among tower grounding system models and among insulator string flashover models whereas rather insignificantly among tower simulation models. There is no systematic variation in computed overvoltages among insulator string flashover models. Single vertical lossless line models and a constant rather than a current dependent resistance are considered, in terms of simulation simplicity and safe design, as satisfactory for simulating transmission line tower and its grounding resistance, respectively, in insulation coordination studies of substations.}
}

Pantelis N. Mikropoulos, Thomas Tsovilis, Z. Politis and A. G. Kagiannas
7th Mediterranean Conference and Exhibition on Power Generation, Transmission, Distribution and Energy Conversion (MedPower 2010), pp. 1-6, Agia Napa, Cyprus, 2010 Nov
[Abstract][BibTex][pdf]

Fast-front overvoltages arising at or transferred to the medium-voltage and low-voltage terminals of a 20/0.4 kV distribution transformer are evaluated with the aid of ATP-EMTP simulations. The effects of lightning crest current and of the transformer and load grounding resistances on the computed overvoltages are investigated. The protection against lightning surges offered by surge protective devices installed close to the low-voltage terminals of the distribution transformer is evaluated. The peak of the overvoltage arising at the medium-voltage terminals of the distribution transformer may exceed the corresponding insulation level even when surge arresters are operating close to the latter. The peak of the overvoltage arising at or transferred to the low-voltage terminals of the distribution transformer may exceed the corresponding insulation level when surge protective devices are not installed. When surge protective devices are installed, the overvoltages are significantly reduced and practically unaffected by transformer and/or load grounding resistances.

@conference{5715996,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and Z. Politis and A. G. Kagiannas},
title={Evaluation of fast-front overvoltages arising at a 20/0.4 kV distribution transformer},
booktitle={7th Mediterranean Conference and Exhibition on Power Generation, Transmission, Distribution and Energy Conversion (MedPower 2010)},
pages={1-6},
address={Agia Napa, Cyprus},
year={2010},
month={11},
date={2010-11-01},
url={http://ieeexplore.ieee.org/document/5715996/},
doi={http://10.1049/cp.2010.0918},
abstract={Fast-front overvoltages arising at or transferred to the medium-voltage and low-voltage terminals of a 20/0.4 kV distribution transformer are evaluated with the aid of ATP-EMTP simulations. The effects of lightning crest current and of the transformer and load grounding resistances on the computed overvoltages are investigated. The protection against lightning surges offered by surge protective devices installed close to the low-voltage terminals of the distribution transformer is evaluated. The peak of the overvoltage arising at the medium-voltage terminals of the distribution transformer may exceed the corresponding insulation level even when surge arresters are operating close to the latter. The peak of the overvoltage arising at or transferred to the low-voltage terminals of the distribution transformer may exceed the corresponding insulation level when surge protective devices are not installed. When surge protective devices are installed, the overvoltages are significantly reduced and practically unaffected by transformer and/or load grounding resistances.}
}

Petros Mavroidis, Pantelis N. Mikropoulos, Constantinos A.Stassinopoulos and Lazaros P. Tsirolias
45th Universities Power Engineering Conference, Cardiff, Wales, 2010 Aug
[Abstract][BibTex][pdf]

Surface corona development in short rod-plane air gaps with a dielectric covered rod under positive lightning and switching impulse voltages is investigated. Discharge activity was recorded through still photographs and oscillograms of the electric field strength at the earthed plane. The gap length, impulse waveshape and material of the dielectric cover were considered as influencing parameters. The extent of development of the surface corona along the dielectric cover becomes greater with increasing applied voltage and field and with decreasing gap length. Also, it is greater for lightning than switching impulse voltages and occurs at significantly lower applied field for the PTFE than epoxy resin cover. Surface corona development, being greatly dependent on the characteristics of the initial corona emerging in the vicinity of the cover tip, affects the inception of the corona emerging at the dielectric cover upper end, thus also breakdown.

@conference{Mavroidis2010UPEC,
author={Petros Mavroidis and Pantelis N. Mikropoulos and Constantinos A.Stassinopoulos and Lazaros P. Tsirolias},
title={Surface corona development in dielectric covered rod-plane air gaps under impulse voltages},
booktitle={45th Universities Power Engineering Conference},
address={Cardiff, Wales},
year={2010},
month={08},
date={2010-08-31},
url={http://ieeexplore.ieee.org/document/5648806/},
abstract={Surface corona development in short rod-plane air gaps with a dielectric covered rod under positive lightning and switching impulse voltages is investigated. Discharge activity was recorded through still photographs and oscillograms of the electric field strength at the earthed plane. The gap length, impulse waveshape and material of the dielectric cover were considered as influencing parameters. The extent of development of the surface corona along the dielectric cover becomes greater with increasing applied voltage and field and with decreasing gap length. Also, it is greater for lightning than switching impulse voltages and occurs at significantly lower applied field for the PTFE than epoxy resin cover. Surface corona development, being greatly dependent on the characteristics of the initial corona emerging in the vicinity of the cover tip, affects the inception of the corona emerging at the dielectric cover upper end, thus also breakdown.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
30th International Conference on Lightning Protection, pp. 1-7, Cagliari, Italy, 2010 Sep
[Abstract][BibTex][pdf]

Shielding performance of overhead transmission lines is evaluated by implementing in shielding analysis a statistical model of lightning attachment. The effects of transmission line parameters, lightning peak current distribution and interception probability on shielding failure flashover rate are investigated and discussed. An application has been made to several overhead transmission lines including typical lines the Hellenic transmission system. Results are compared with that obtained by employing in shielding analysis some commonly used electrogeometric models. The estimated shielding failure flashover rate, showing a great variability among lightning attachment models, is much more sensitive to transmission line geometry for the IEEE Standard 1243:1997, which yields the highest values among models. For all lightning attachment models employed in shielding analysis the estimated shielding failure flashover rate of several overhead transmission lines is in qualitative agreement but in quantitative inconsistency with field data.

@conference{Mikropoulos2010ICLP,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Lightning interception probability and shielding performance of overhead transmission lines},
booktitle={30th International Conference on Lightning Protection},
pages={1-7},
address={Cagliari, Italy},
year={2010},
month={09},
date={2010-09-13},
url={http://ieeexplore.ieee.org/document/7845912/},
doi={http://10.1109/ICLP.2010.7845912},
abstract={Shielding performance of overhead transmission lines is evaluated by implementing in shielding analysis a statistical model of lightning attachment. The effects of transmission line parameters, lightning peak current distribution and interception probability on shielding failure flashover rate are investigated and discussed. An application has been made to several overhead transmission lines including typical lines the Hellenic transmission system. Results are compared with that obtained by employing in shielding analysis some commonly used electrogeometric models. The estimated shielding failure flashover rate, showing a great variability among lightning attachment models, is much more sensitive to transmission line geometry for the IEEE Standard 1243:1997, which yields the highest values among models. For all lightning attachment models employed in shielding analysis the estimated shielding failure flashover rate of several overhead transmission lines is in qualitative agreement but in quantitative inconsistency with field data.}
}

Pantelis N. Mikropoulos, Thomas Tsovilis, Iordanis Manousaridis, Georgios Laloumis and Asterios Dramis
MedPower 2010, pp. 1-5, Agia Napa, Cyprus, 2010 Nov
[Abstract][BibTex][pdf]

Lightning risk assessment of a 170 kV GIS substation connected to the Hellenic transmission system through underground cables has been performed. Fast-front overvoltages impinging on the substation due to shielding failure and backflashover of the incoming overhead transmission lines have been computed with the aid of ATPEMTP simulations and evaluated with respect to the insulation level of the substation equipment. The mean time between failures of the substation is greatly reduced for higher tower grounding resistance and varies significantly among lightning attachment models employed for the evaluation of the lightning performance of the incoming overhead transmission lines. Implementation of the IEEE Std 1243:1997 in lightning risk assessment of substations imposes high requirements on protection against incoming fast-front overvoltage surges.

@conference{Mikropoulos2010medpower,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and Iordanis Manousaridis and Georgios Laloumis and Asterios Dramis},
title={Lightning risk assessment of a 170 kV GIS substation connected to the Hellenic Transmission System through underground cables},
booktitle={MedPower 2010},
pages={1-5},
address={Agia Napa, Cyprus},
year={2010},
month={11},
date={2010-11-07},
url={http://ieeexplore.ieee.org/document/5715976/},
doi={http://10.1049/cp.2010.0898},
abstract={Lightning risk assessment of a 170 kV GIS substation connected to the Hellenic transmission system through underground cables has been performed. Fast-front overvoltages impinging on the substation due to shielding failure and backflashover of the incoming overhead transmission lines have been computed with the aid of ATPEMTP simulations and evaluated with respect to the insulation level of the substation equipment. The mean time between failures of the substation is greatly reduced for higher tower grounding resistance and varies significantly among lightning attachment models employed for the evaluation of the lightning performance of the incoming overhead transmission lines. Implementation of the IEEE Std 1243:1997 in lightning risk assessment of substations imposes high requirements on protection against incoming fast-front overvoltage surges.}
}

Pantelis N. Mikropoulos and Lazaros C. Tzimkas
45th Universities Power Engineering Conference, pp. 1-5, Cardiff, Wales, 2010 Aug
[Abstract][BibTex][pdf]

Knowledge of surface streamer characteristics as influenced by the insulating surface and atmospheric conditions is important for the better understanding of the flashover mechanism, hence also for an efficient design of insulating systems under various environmental conditions. In the present study positive streamer propagation along PTFE and Nylon cylindrical insulating surfaces is investigated in a uniform field in air under variable absolute humidity. Streamers were initiated by applying at an auxiliary needle electrode, located at the earthed anode plane electrode, a pulse voltage variable in amplitude and propagated along an insulating surface towards the upper plane electrode, which was stressed by negative DC voltage. The electric fields required for threshold and stable surface streamer propagation and the velocity of stable streamer propagation, being higher than that for air alone and also slightly higher for PTFE than Nylon insulator, increase with absolute humidity. Empirical relationships have been derived, accurately expressing the effects of absolute humidity on surface streamer propagation field and velocity. Results are compared to that obtained in the case of streamer propagation in air alone based on previous work.

@conference{mikropoulos2010UPEC,
author={Pantelis N. Mikropoulos and Lazaros C. Tzimkas},
title={Influence of humidity on surface streamer propagation in a uniform field in air},
booktitle={45th Universities Power Engineering Conference},
pages={1-5},
address={Cardiff, Wales},
year={2010},
month={08},
date={2010-08-31},
url={http://ieeexplore.ieee.org/document/5650040/},
abstract={Knowledge of surface streamer characteristics as influenced by the insulating surface and atmospheric conditions is important for the better understanding of the flashover mechanism, hence also for an efficient design of insulating systems under various environmental conditions. In the present study positive streamer propagation along PTFE and Nylon cylindrical insulating surfaces is investigated in a uniform field in air under variable absolute humidity. Streamers were initiated by applying at an auxiliary needle electrode, located at the earthed anode plane electrode, a pulse voltage variable in amplitude and propagated along an insulating surface towards the upper plane electrode, which was stressed by negative DC voltage. The electric fields required for threshold and stable surface streamer propagation and the velocity of stable streamer propagation, being higher than that for air alone and also slightly higher for PTFE than Nylon insulator, increase with absolute humidity. Empirical relationships have been derived, accurately expressing the effects of absolute humidity on surface streamer propagation field and velocity. Results are compared to that obtained in the case of streamer propagation in air alone based on previous work.}
}

Pantelis N. Mikropoulos, Thomas Tsovilis and D.E Zlitidis
45th Universities Power Engineering Conference, pp. 1-6, Cardiff, Wales, 2010 Aug
[Abstract][BibTex][pdf]

A useful tool for the evaluation of the lightning performance of overhead transmission lines has been developed in Matlab. The software (LPTL) runs as a Microsoft Windows application and features a user-friendly graphics interface. For given overhead transmission line parameters, ground flash density and lightning crest current distribution the lightning performance of the line can be easily evaluated according to several lightning attachment models. Through an application to a typical 400 kV double circuit line of the Hellenic transmission system the effects of lightning attachment model and lightning crest current distribution on the lightning performance of the line are quantified. Generally, electrogeometric models yield the highest shielding failure flashover rate whereas the lowest backflashover rate. As the median value of the lightning crest current distribution increases the shielding failure flashover rate decreases whereas backflashover rate increases. The application software is a useful tool for utilities and can also be used for educational purposes in high voltage engineering courses.

@conference{Mikropoulos2010UPECTsovilis,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and D.E Zlitidis},
title={Software development for the evaluation of the lightning performance of overhead transmission lines},
booktitle={45th Universities Power Engineering Conference},
pages={1-6},
address={Cardiff, Wales},
year={2010},
month={08},
date={2010-08-31},
url={http://ieeexplore.ieee.org/document/5648865/?tp=&arnumber=5648865},
abstract={A useful tool for the evaluation of the lightning performance of overhead transmission lines has been developed in Matlab. The software (LPTL) runs as a Microsoft Windows application and features a user-friendly graphics interface. For given overhead transmission line parameters, ground flash density and lightning crest current distribution the lightning performance of the line can be easily evaluated according to several lightning attachment models. Through an application to a typical 400 kV double circuit line of the Hellenic transmission system the effects of lightning attachment model and lightning crest current distribution on the lightning performance of the line are quantified. Generally, electrogeometric models yield the highest shielding failure flashover rate whereas the lowest backflashover rate. As the median value of the lightning crest current distribution increases the shielding failure flashover rate decreases whereas backflashover rate increases. The application software is a useful tool for utilities and can also be used for educational purposes in high voltage engineering courses.}
}

Pantelis N. Mikropoulos and Vasileios Zagkanas
45th Universities Power Engineering Conference, pp. 1-5, Cardiff, Wales, 2010 Aug
[Abstract][BibTex][pdf]

Corona discharge has attracted much interest among researchers as it has a lot of applications in industry and a number of effects on power systems. In the coaxial cylindrical electrode arrangement in air, simulating many practical applications, corona inception depends on the electric field strength around the inner conductor and on atmospheric conditions. In the present study a computational method for the estimation of the negative corona inception field strength in the coaxial cylindrical electrode arrangement under variable air density is presented. The method is based on streamer theory, involves Hartmann\'s expression for the effective ionization coefficient and considers the effect of the space charge field on avalanche growth. The computed negative corona inception field strength is in good agreement with literature experimental data for an avalanche number varying with the product of relative air density and inner conductor radius. Streamer formation occurs rather through a multi-avalanche process, assisted by subsidiary avalanches owing to secondary ionization processes at the cathode and in air. Peek\'s empirical formula for estimating corona inception field strength in the coaxial cylindrical electrode arrangement in air agrees satisfactorily with literature experimental data referring to negative corona inception.

@conference{Mikropoulos2010Zagkanas,
author={Pantelis N. Mikropoulos and Vasileios Zagkanas},
title={Computation of negative corona inception field strength in the coaxial cylindrical electrode arrangement under variable air density},
booktitle={45th Universities Power Engineering Conference},
pages={1-5},
address={Cardiff, Wales},
year={2010},
month={08},
date={2010-08-31},
url={http://ieeexplore.ieee.org/document/5649939/},
abstract={Corona discharge has attracted much interest among researchers as it has a lot of applications in industry and a number of effects on power systems. In the coaxial cylindrical electrode arrangement in air, simulating many practical applications, corona inception depends on the electric field strength around the inner conductor and on atmospheric conditions. In the present study a computational method for the estimation of the negative corona inception field strength in the coaxial cylindrical electrode arrangement under variable air density is presented. The method is based on streamer theory, involves Hartmann\\'s expression for the effective ionization coefficient and considers the effect of the space charge field on avalanche growth. The computed negative corona inception field strength is in good agreement with literature experimental data for an avalanche number varying with the product of relative air density and inner conductor radius. Streamer formation occurs rather through a multi-avalanche process, assisted by subsidiary avalanches owing to secondary ionization processes at the cathode and in air. Peek\\'s empirical formula for estimating corona inception field strength in the coaxial cylindrical electrode arrangement in air agrees satisfactorily with literature experimental data referring to negative corona inception.}
}

2009

Journal Articles

Pantelis N. Mikropoulos and Thomas Tsovilis
IEEE Trans. on Power Delivery, 24, (2), pp. 863-873, 2009 Feb
[Abstract][BibTex][pdf]

A comprehensive analysis on shielding against lightning is presented. For an air terminal, the design of its protection zone and collection volume is introduced on the basis of simple working relationships which take into account, besides lightning parameters and air terminal height, the interception probability and use as a reference the striking distance to earth surface. The analysis, based on lightning scale-model experimental results, includes a comparison with previously reported models and field data; a satisfactory agreement is found. Lightning incidence calculations have been performed on the basis of lightning interception probability; a close conformity between the actual and calculated number of lightning strikes exists. Both striking distance and interception radius and their statistical nature should be considered for an accurate description of lightning interception.

@article{Mikropoulos2009,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Interception probability and shielding against lightning},
journal={IEEE Trans. on Power Delivery},
volume={24},
number={2},
pages={863-873},
year={2009},
month={02},
date={2009-02-13},
url={http://ieeexplore.ieee.org/document/4783088/?tp=&arnumber=4783088&queryText%3Dinterception+probability},
doi={http://10.1109/TPWRD.2008.2002979},
issn={0885-8977},
abstract={A comprehensive analysis on shielding against lightning is presented. For an air terminal, the design of its protection zone and collection volume is introduced on the basis of simple working relationships which take into account, besides lightning parameters and air terminal height, the interception probability and use as a reference the striking distance to earth surface. The analysis, based on lightning scale-model experimental results, includes a comparison with previously reported models and field data; a satisfactory agreement is found. Lightning incidence calculations have been performed on the basis of lightning interception probability; a close conformity between the actual and calculated number of lightning strikes exists. Both striking distance and interception radius and their statistical nature should be considered for an accurate description of lightning interception.}
}

2009

Conference Papers

Pantelis N. Mikropoulos Thomas Tsovilis
IEEE Transactions on Power Delivery, pp. 863-873, Uppsala, 2009 Apr
[Abstract][BibTex][pdf]

A comprehensive analysis on shielding against lightning is presented. For an air terminal, the design of its protection zone and collection volume is introduced on the basis of simple working relationships which take into account, besides lightning parameters and air terminal height, the interception probability and use as a reference the striking distance to earth surface. The analysis, based on lightning scale-model experimental results, includes a comparison with previously reported models and field data; a satisfactory agreement is found. Lightning incidence calculations have been performed on the basis of lightning interception probability; a close conformity between the actual and calculated number of lightning strikes exists. Both striking distance and interception radius and their statistical nature should be considered for an accurate description of lightning interception.

@conference{4783088,
author={Pantelis N. Mikropoulos Thomas Tsovilis},
title={Interception Probability and Shielding Against Lightning},
booktitle={IEEE Transactions on Power Delivery},
pages={863-873},
address={Uppsala},
year={2009},
month={04},
date={2009-04-01},
url={http://ieeexplore.ieee.org/document/4783088/},
doi={http://10.1109/TPWRD.2008.2002979},
abstract={A comprehensive analysis on shielding against lightning is presented. For an air terminal, the design of its protection zone and collection volume is introduced on the basis of simple working relationships which take into account, besides lightning parameters and air terminal height, the interception probability and use as a reference the striking distance to earth surface. The analysis, based on lightning scale-model experimental results, includes a comparison with previously reported models and field data; a satisfactory agreement is found. Lightning incidence calculations have been performed on the basis of lightning interception probability; a close conformity between the actual and calculated number of lightning strikes exists. Both striking distance and interception radius and their statistical nature should be considered for an accurate description of lightning interception.}
}

Lazaros Lazaridis and Pantelis N. Mikropoulos
16th International Symposium on High Voltage Engineering, Cape Town, South Africa, 2009 Aug
[Abstract][BibTex][pdf]

Positive impulse flashover of a short rod-plane gap bridged by cylindrical insulating surfaces has been investigated in a wide range of absolute humidity variation. Silicon rubber and porcelain insulators with a smooth surface were inserted parallel to the rod, which was stressed by positive standard lightning and switching impulse voltages; air alone was regarded as reference. A capacitive probe positioned at the centre of the earthed plane connected to a digital oscilloscope enabled the monitoring of the electric field strength at the plane. Flashover probability distributions were obtained and the time and voltage to flashover were measured. In the presence of an insulating surface the dielectric strength is reduced under lightning impulses but increased under switching impulses when compared to the case of air alone. Increasing humidity results in shorter time to flashover and increased surface dielectric strength, however, these humidity effects depend upon the trajectory of the spark channel. With the only exception of the porcelain surface under lightning impulses, the discharge develops away from the insulating surface in free air; hence, the effect of humidity on flashover is similar to that observed in the simple air gap. Where flashover occurs close to the insulating surface the atmospheric conditions correction procedure according to IEC 60060-1:1989 should be optimised by employing a reduced humidity coefficient.

@conference{Lazaridis2009,
author={Lazaros Lazaridis and Pantelis N. Mikropoulos},
title={Influence of humidity on positive impulse flashover along cylindrical insulating surfaces bridging a short rod-plane gap},
booktitle={16th International Symposium on High Voltage Engineering},
address={Cape Town, South Africa},
year={2009},
month={08},
date={2009-08-01},
url={https://www.researchgate.net/publication/255948672_Influence_of_humidity_on_positive_impulse_flashover_along_cylindrical_insulating_surfaces_bridging_a_short_rod-plane_gap},
abstract={Positive impulse flashover of a short rod-plane gap bridged by cylindrical insulating surfaces has been investigated in a wide range of absolute humidity variation. Silicon rubber and porcelain insulators with a smooth surface were inserted parallel to the rod, which was stressed by positive standard lightning and switching impulse voltages; air alone was regarded as reference. A capacitive probe positioned at the centre of the earthed plane connected to a digital oscilloscope enabled the monitoring of the electric field strength at the plane. Flashover probability distributions were obtained and the time and voltage to flashover were measured. In the presence of an insulating surface the dielectric strength is reduced under lightning impulses but increased under switching impulses when compared to the case of air alone. Increasing humidity results in shorter time to flashover and increased surface dielectric strength, however, these humidity effects depend upon the trajectory of the spark channel. With the only exception of the porcelain surface under lightning impulses, the discharge develops away from the insulating surface in free air; hence, the effect of humidity on flashover is similar to that observed in the simple air gap. Where flashover occurs close to the insulating surface the atmospheric conditions correction procedure according to IEC 60060-1:1989 should be optimised by employing a reduced humidity coefficient.}
}

Petros Mavroidis, Pantelis N. Mikropoulos, Constantine A. Stassinopoulos and M Zinonos
16th International Symposium on High Voltage Engineering, Cape Town, South Africa, 2009 Aug
[Abstract][BibTex][pdf]

The breakdown mechanism of short rod-plane gaps with a dielectric covered rod under positive standard lightning and switching impulse voltages is investigated. Two different dielectric covers, made of PTFE and epoxy resin, were used and the simple air gaps with a bare rod were regarded as reference. Breakdown probability distributions were obtained and the time and voltage to breakdown were measured. Under lightning impulses the increase of the dielectric strength due to the dielectric cover is little dependent upon the cover material and only slightly higher to that of a simple air gap including the cover length. Under switching impulses this increase is greater; the presence of a dielectric cover results in a reduced effect of the impulse waveshape on the dielectric strength. Most of the times, the spark channel develops along the dielectric cover under lightning impulses whereas, under switching impulses it develops either partly or solely in free air away from the cover surface. The increase in the dielectric strength of the gaps due to the dielectric cover is associated with the effects on the discharge development pattern of the coronas preceding breakdown.

@conference{Mavroidis2009,
author={Petros Mavroidis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos and M Zinonos},
title={Impulse breakdown of short rod-plane gaps with rod covered with different dielectric materials},
booktitle={16th International Symposium on High Voltage Engineering},
address={Cape Town, South Africa},
year={2009},
month={08},
date={2009-08-01},
url={https://www.researchgate.net/publication/255948587_Impulse_breakdown_of_short_rod-plane_gaps_with_rod_covered_with_different_dielectric_materials},
abstract={The breakdown mechanism of short rod-plane gaps with a dielectric covered rod under positive standard lightning and switching impulse voltages is investigated. Two different dielectric covers, made of PTFE and epoxy resin, were used and the simple air gaps with a bare rod were regarded as reference. Breakdown probability distributions were obtained and the time and voltage to breakdown were measured. Under lightning impulses the increase of the dielectric strength due to the dielectric cover is little dependent upon the cover material and only slightly higher to that of a simple air gap including the cover length. Under switching impulses this increase is greater; the presence of a dielectric cover results in a reduced effect of the impulse waveshape on the dielectric strength. Most of the times, the spark channel develops along the dielectric cover under lightning impulses whereas, under switching impulses it develops either partly or solely in free air away from the cover surface. The increase in the dielectric strength of the gaps due to the dielectric cover is associated with the effects on the discharge development pattern of the coronas preceding breakdown.}
}

Pantelis N. Mikropoulos, Lazaros C. Tzimkas, T Giannopoulos and P Tsintikidis
"Positive streamer propagation along profiled insulating surfaces with room temperature vulcanized silicone rubber coatings"
16th International Symposium on High Voltage Engineering, Cape Town, South Africa, 2009 Aug
[Abstract][BibTex]

Results on positive streamer propagation along smooth and profiled cylindrical insulating surfaces coated with different room temperature vulcanized (RTV) silicon rubber coatings are presented; nylon specimens were regarded as reference. Experiments were carried out in a three electrode arrangement consisting of a 12 cm long parallel-plane gap with an auxiliary needle in the earthed anode. Streamers were initiated by applying at the needle electrode a pulse voltage variable in amplitude and propagated over the insulators towards the upper plane electrode which was stressed by a negative DC voltage. When streamers propagate stably along an insulating smooth surface the propagation field and the associated velocity are both higher for RTV coated than bare insulators, however when they propagate along a profiled insulating surface with a step or a shed this reverses itself. The effect of a step or a shed on an insulating surface is to increase propagation field, with the only exception where a streamer crosses part of the gap in air alone, and to reduce the velocity of propagation at all applied fields. The increase of streamer propagation field due to surface profile is less marked for RTV coated than bare insulating surfaces whereas the reduction of propagation velocity is more pronounced.

@conference{Mikropoulos2009ISHVE,
author={Pantelis N. Mikropoulos and Lazaros C. Tzimkas and T Giannopoulos and P Tsintikidis},
title={Positive streamer propagation along profiled insulating surfaces with room temperature vulcanized silicone rubber coatings},
booktitle={16th International Symposium on High Voltage Engineering},
address={Cape Town, South Africa},
year={2009},
month={08},
date={2009-08-01},
abstract={Results on positive streamer propagation along smooth and profiled cylindrical insulating surfaces coated with different room temperature vulcanized (RTV) silicon rubber coatings are presented; nylon specimens were regarded as reference. Experiments were carried out in a three electrode arrangement consisting of a 12 cm long parallel-plane gap with an auxiliary needle in the earthed anode. Streamers were initiated by applying at the needle electrode a pulse voltage variable in amplitude and propagated over the insulators towards the upper plane electrode which was stressed by a negative DC voltage. When streamers propagate stably along an insulating smooth surface the propagation field and the associated velocity are both higher for RTV coated than bare insulators, however when they propagate along a profiled insulating surface with a step or a shed this reverses itself. The effect of a step or a shed on an insulating surface is to increase propagation field, with the only exception where a streamer crosses part of the gap in air alone, and to reduce the velocity of propagation at all applied fields. The increase of streamer propagation field due to surface profile is less marked for RTV coated than bare insulating surfaces whereas the reduction of propagation velocity is more pronounced.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
IEEE PowerTech 2009, pp. 1-8, Bucharest, Romania, 2009 Jul
[Abstract][BibTex][pdf]

General relationships for the estimation of the maximum shielding failure current of overhead transmission lines have been derived by performing shielding analysis on the basis of several lightning attachment models including a recently introduced statistical one. The interdependence of maximum shielding failure current, transmission line geometry and factors employed in lightning attachment models is discussed through an application to typical 150 kV and 400 kV lines of the Hellenic transmission system. The maximum shielding failure current depends on transmission line geometry and shows a great variability among the lightning attachment models that are used in shielding analysis; electrogeometric models, thus also the IEEE Standard 1243:1997, yield higher values. These results are of great importance when considering that the maximum shielding failure current of transmission lines, besides being employed in estimating their shielding failure flashover rate, is an important parameter for insulation coordination studies.

@conference{Mikropoulos2009PowerTech,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Lightning attachment models and maximum shielding failure current: Application to transmission lines},
booktitle={IEEE PowerTech 2009},
pages={1-8},
address={Bucharest, Romania},
year={2009},
month={07},
date={2009-07-01},
url={http://ieeexplore.ieee.org/document/5282095/},
doi={http://10.1109/PTC.2009.5282095},
abstract={General relationships for the estimation of the maximum shielding failure current of overhead transmission lines have been derived by performing shielding analysis on the basis of several lightning attachment models including a recently introduced statistical one. The interdependence of maximum shielding failure current, transmission line geometry and factors employed in lightning attachment models is discussed through an application to typical 150 kV and 400 kV lines of the Hellenic transmission system. The maximum shielding failure current depends on transmission line geometry and shows a great variability among the lightning attachment models that are used in shielding analysis; electrogeometric models, thus also the IEEE Standard 1243:1997, yield higher values. These results are of great importance when considering that the maximum shielding failure current of transmission lines, besides being employed in estimating their shielding failure flashover rate, is an important parameter for insulation coordination studies.}
}

Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
44th Universities Power Engineering Conference, pp. 1-4, Glasgow, Scotland, 2009 Sep
[Abstract][BibTex][pdf]

Positive corona inception in dielectric covered rod-plane air gaps stressed by lightning and switching impulse voltages is investigated. Corona inception probability distributions were obtained and the salient characteristics of the corona discharge, namely inception time and voltage, were measured as influenced by the gap length, waveshape of the applied impulse voltage and by the cover material, namely PTFE and epoxy resin. The simple air gaps with a bare rod were regarded as reference. The corona inception voltage increases significantly due to the insulating cover, especially for the PTFE cover under switching voltages. Electric field computations were performed. The corona inception field is, contrary to the inception voltage, lower for the dielectric covered rod-plane than simple rod-plane air gaps; an assisting mechanism for initiatory electrons production is suggested to explain this behaviour.

@conference{Mikropoulos2009Stassinopoulos,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Impulse corona inception in dielectric covered rod-plane air gaps},
booktitle={44th Universities Power Engineering Conference},
pages={1-4},
address={Glasgow, Scotland},
year={2009},
month={09},
date={2009-09-01},
url={http://ieeexplore.ieee.org/document/5429436/},
abstract={Positive corona inception in dielectric covered rod-plane air gaps stressed by lightning and switching impulse voltages is investigated. Corona inception probability distributions were obtained and the salient characteristics of the corona discharge, namely inception time and voltage, were measured as influenced by the gap length, waveshape of the applied impulse voltage and by the cover material, namely PTFE and epoxy resin. The simple air gaps with a bare rod were regarded as reference. The corona inception voltage increases significantly due to the insulating cover, especially for the PTFE cover under switching voltages. Electric field computations were performed. The corona inception field is, contrary to the inception voltage, lower for the dielectric covered rod-plane than simple rod-plane air gaps; an assisting mechanism for initiatory electrons production is suggested to explain this behaviour.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
16th International Symposium on High Voltage Engineering, Cape Town, South Africa, 2009 Aug
[Abstract][BibTex][pdf]

Lightning is the main cause of transmission line outages affecting reliability of power supply thus, consequently, resulting in economic losses. A statistical method for lightning incidence calculations in transmission lines is introduced. Simple expressions for the estimation of an expected range of lightning strikes to a transmission line depending on interception probability distribution have been obtained, based on a recently proposed statistical lightning attachment model derived from scale model experiments. The expected number of lightning strikes depends, besides transmission line geometry, on lightning stroke current distribution and interception probability. The results of the statistical method have been compared with those obtained by employing other models from literature, including that suggested by the IEEE Std. 1243:1997, in lightning incidence calculations, and with field observation data; a satisfactory agreement has been shown to exist. Results on lightning incidence calculations are further discussed through an application to typical 150 kV and 400 kV lines of the Hellenic transmission system.

@conference{Mikropoulos2009Tsovilis,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={A statistical method for lightning incidence calculations in transmission lines},
booktitle={16th International Symposium on High Voltage Engineering},
address={Cape Town, South Africa},
year={2009},
month={08},
date={2009-08-01},
url={https://www.researchgate.net/publication/235986213_A_statistical_method_for_lightning_incidence_calculations_in_transmission_lines},
abstract={Lightning is the main cause of transmission line outages affecting reliability of power supply thus, consequently, resulting in economic losses. A statistical method for lightning incidence calculations in transmission lines is introduced. Simple expressions for the estimation of an expected range of lightning strikes to a transmission line depending on interception probability distribution have been obtained, based on a recently proposed statistical lightning attachment model derived from scale model experiments. The expected number of lightning strikes depends, besides transmission line geometry, on lightning stroke current distribution and interception probability. The results of the statistical method have been compared with those obtained by employing other models from literature, including that suggested by the IEEE Std. 1243:1997, in lightning incidence calculations, and with field observation data; a satisfactory agreement has been shown to exist. Results on lightning incidence calculations are further discussed through an application to typical 150 kV and 400 kV lines of the Hellenic transmission system.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
44th Universities Power Engineering Conference, pp. 1-5, Glasgow, Scotland, 2009 Sep
[Abstract][BibTex][pdf]

General relationships for the estimation of the perfect shielding angle of overhead transmission lines have been derived by performing shielding analysis on the basis of several lightning attachment models, including a recently introduced statistical one. The interdependence of perfect shielding angle, transmission line height and minimum current causing flashover of insulation is demonstrated as influenced by the lightning attachment model employed in shielding analysis. There is a great variability in perfect shielding angle among lightning attachment models; this is demonstrated for 150 kV and 400 kV lines of the Hellenic transmission system. The applicability of lightning attachment models in perfect shielding angle calculations is evaluated based on the shielding performance of transmission lines; the IEEE Std 1243:1997 yields consistent results with respect to the shielding performance of the lines.

@conference{Mikropoulos2009UPEC,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Lightning attachment models and perfect shielding angle of transmission lines},
booktitle={44th Universities Power Engineering Conference},
pages={1-5},
address={Glasgow, Scotland},
year={2009},
month={09},
date={2009-09-01},
url={http://ieeexplore.ieee.org/document/5429394/},
abstract={General relationships for the estimation of the perfect shielding angle of overhead transmission lines have been derived by performing shielding analysis on the basis of several lightning attachment models, including a recently introduced statistical one. The interdependence of perfect shielding angle, transmission line height and minimum current causing flashover of insulation is demonstrated as influenced by the lightning attachment model employed in shielding analysis. There is a great variability in perfect shielding angle among lightning attachment models; this is demonstrated for 150 kV and 400 kV lines of the Hellenic transmission system. The applicability of lightning attachment models in perfect shielding angle calculations is evaluated based on the shielding performance of transmission lines; the IEEE Std 1243:1997 yields consistent results with respect to the shielding performance of the lines.}
}

Pantelis N. Mikropoulos and Vasileios Zagkanas
16th International Symposium on High Voltage Engineering, Cape Town, South Africa, 2009 Aug
[Abstract][BibTex][pdf]

Corona discharge has many practical applications, thus it has been studied extensively experimentally as well through modelling in many electrode arrangements. In the present study a computational method for the estimation of the positive corona inception field strength in the coaxial cylindrical electrode arrangement in air is presented. It is based on streamer theory and involves Hartmann’s expression for the field dependent effective ionization coefficient and the known distribution of the geometric electric field. A very good agreement with literature experimental data referring to wire-cylinder air gaps has been observed for an avalanche number of 10^4 (ionization integral ~9.2) in a wide range of wire radii and under variable atmospheric conditions. A simple absolute humidity correction factor has been introduced in Peek’s formula, allowing for an accurate estimation of the corona inception field strength under variable humidity. A computational method for positive corona inception in the coaxial cylindrical electrode arrangement in air under variable atmospheric conditions (PDF

@conference{Mikropoulos2009Zagkanas,
author={Pantelis N. Mikropoulos and Vasileios Zagkanas},
title={A computational method for positive corona inception in the coaxial cylindrical electrode arrangement in air under variable atmospheric conditions},
booktitle={16th International Symposium on High Voltage Engineering},
address={Cape Town, South Africa},
year={2009},
month={08},
date={2009-08-01},
url={https://www.researchgate.net/publication/235799476_A_computational_method_for_positive_corona_inception_in_the_coaxial_cylindrical_electrode_arrangement_in_air_under_variable_atmospheric_conditions},
abstract={Corona discharge has many practical applications, thus it has been studied extensively experimentally as well through modelling in many electrode arrangements. In the present study a computational method for the estimation of the positive corona inception field strength in the coaxial cylindrical electrode arrangement in air is presented. It is based on streamer theory and involves Hartmann’s expression for the field dependent effective ionization coefficient and the known distribution of the geometric electric field. A very good agreement with literature experimental data referring to wire-cylinder air gaps has been observed for an avalanche number of 10^4 (ionization integral ~9.2) in a wide range of wire radii and under variable atmospheric conditions. A simple absolute humidity correction factor has been introduced in Peek’s formula, allowing for an accurate estimation of the corona inception field strength under variable humidity. A computational method for positive corona inception in the coaxial cylindrical electrode arrangement in air under variable atmospheric conditions (PDF}
}

2008

Journal Articles

Lazaros Lazaridis and Pantelis N. Mikropoulos
IEEE Trans. on Dielectrics and Electrical Insulation, 15, (3), pp. 694-700, 2008 Jul
[Abstract][BibTex][pdf]

Results are presented concerning discharge development and breakdown in air and along cylindrical insulating surfaces in a 12 cm long rod-plane gap under positive switching impulse voltages. Discharge development and breakdown probability curves were obtained. The times taken for the discharge to cross the full gap and for breakdown were measured and the corresponding instantaneous voltages were calculated. The gap was over stressed so as to study the effect of the applied peak voltage on the discharge characteristics. In the presence of an insulating surface the discharge consists of a \'surface\' and an \'air\' component; the former hinders the development of the latter. At voltages causing 50% breakdown probability only the \'air\' component traverses the full gap and breakdown occurs always in free air. For higher applied voltages the \'surface\' component may become able to cross the full gap and breakdown may occur also along the insulating surface; under these conditions both the arrival of the discharge at the plane and breakdown occur at shorter times and at lower voltages than in air alone, depending on insulating surface material.

@article{Lazaridis2008,
author={Lazaros Lazaridis and Pantelis N. Mikropoulos},
title={Flashover along cylindrical insulating surfaces in a non-uniform field under positive switching impulse voltages},
journal={IEEE Trans. on Dielectrics and Electrical Insulation},
volume={15},
number={3},
pages={694-700},
year={2008},
month={07},
date={2008-07-13},
url={http://ieeexplore.ieee.org/document/4543106/?arnumber=4543106},
doi={http://10.1109/TDEI.2008.4543106},
issn={1070-9878},
abstract={Results are presented concerning discharge development and breakdown in air and along cylindrical insulating surfaces in a 12 cm long rod-plane gap under positive switching impulse voltages. Discharge development and breakdown probability curves were obtained. The times taken for the discharge to cross the full gap and for breakdown were measured and the corresponding instantaneous voltages were calculated. The gap was over stressed so as to study the effect of the applied peak voltage on the discharge characteristics. In the presence of an insulating surface the discharge consists of a \\'surface\\' and an \\'air\\' component; the former hinders the development of the latter. At voltages causing 50% breakdown probability only the \\'air\\' component traverses the full gap and breakdown occurs always in free air. For higher applied voltages the \\'surface\\' component may become able to cross the full gap and breakdown may occur also along the insulating surface; under these conditions both the arrival of the discharge at the plane and breakdown occur at shorter times and at lower voltages than in air alone, depending on insulating surface material.}
}

Pantelis N. Mikropoulos
IET Sci. Meas. Technol., 2, (4), pp. 187-195, 2008 Jun
[Abstract][BibTex][pdf]

Results concerning streamer propagation along cylindrical insulators coated with room-temperature-vulcanised silicon rubber coatings are presented. Experiments were carried out in a three-electrode arrangement consisting of a 12?cm long parallel-plane gap with an auxiliary needle in the earthed anode. Insulators could be inserted between the plane electrodes adjacent to them and almost in contact with the needle. Positive streamers were initiated by applying at the needle electrode a pulse voltage variable in amplitude and propagated over the insulators towards the upper plane electrode which was stressed by a negative DC voltage. The streamer propagation field at conditions from threshold up to the stable streamer propagation and the associated velocity were measured. The dependence of the streamer velocity on the electric field was investigated with the type of coating as parameter. Both the streamer propagation field and the associated velocity of propagation are higher, when a streamer propagates along the coated insulators, than the reference bare nylon insulator depending on the coating employed. Streamers might be used as an effective probe for studying the surface dielectric behaviour of coated insulators hence also for evaluating the various coatings used to improve the contamination performance of outdoor high-voltage insulators.

@article{Mikropoulos2008,
author={Pantelis N. Mikropoulos},
title={Streamer propagation along room-temperature-vulcanised silicon-rubber-coated cylindrical insulators},
journal={IET Sci. Meas. Technol.},
volume={2},
number={4},
pages={187-195},
year={2008},
month={06},
date={2008-06-01},
url={http://digital-library.theiet.org/content/journals/10.1049/iet-smt_20070062},
abstract={Results concerning streamer propagation along cylindrical insulators coated with room-temperature-vulcanised silicon rubber coatings are presented. Experiments were carried out in a three-electrode arrangement consisting of a 12?cm long parallel-plane gap with an auxiliary needle in the earthed anode. Insulators could be inserted between the plane electrodes adjacent to them and almost in contact with the needle. Positive streamers were initiated by applying at the needle electrode a pulse voltage variable in amplitude and propagated over the insulators towards the upper plane electrode which was stressed by a negative DC voltage. The streamer propagation field at conditions from threshold up to the stable streamer propagation and the associated velocity were measured. The dependence of the streamer velocity on the electric field was investigated with the type of coating as parameter. Both the streamer propagation field and the associated velocity of propagation are higher, when a streamer propagates along the coated insulators, than the reference bare nylon insulator depending on the coating employed. Streamers might be used as an effective probe for studying the surface dielectric behaviour of coated insulators hence also for evaluating the various coatings used to improve the contamination performance of outdoor high-voltage insulators.}
}

Pantelis N. Mikropoulos, Constantine A. Stassinopoulos and Barbara C. Sarigiannidou
IEEE Trans. on Dielectrics and Electrical Insulation, 15, (2), pp. 416-425, 2008 Apr
[Abstract][BibTex][pdf]

The influence of humidity on streamer propagation at conditions from the threshold for propagation to those for streamer-induced breakdown was investigated in a uniform field in air at atmospheric pressure. Experiments were carried out in a three electrode arrangement consisting of a 12 cm long parallel-plane gap, with an auxiliary needle in the earthed anode. Positive streamers were initiated by applying at the needle electrode a pulse voltage which varied in amplitude. These propagated towards the upper plane electrode which was stressed by a negative dc voltage. Under natural atmospheric conditions, propagation and breakdown probability curves were obtained for several values of absolute humidity in the range between 5 and 22 g/m3. Thus, distributions of the electric field required for streamer propagation and breakdown were obtained and the associated velocity of propagation and time to breakdown were measured. Besides humidity, the amplitude of the voltage used for streamer initiation and the ambient electric field were considered as influencing parameters on streamer properties. Empirical equations are presented expressing the effects of the above parameters on the intrinsic streamer properties. A comparison with previous work in the literature is made and this leads to the conclusion that the influence of humidity on streamer propagation and breakdown can be placed in a sounder quantitative basis.

@article{Mikropoulos2008TDEI,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos and Barbara C. Sarigiannidou},
title={Positive streamer propagation and breakdown in air: the influence of humidity},
journal={IEEE Trans. on Dielectrics and Electrical Insulation},
volume={15},
number={2},
pages={416-425},
year={2008},
month={04},
date={2008-04-03},
url={http://ieeexplore.ieee.org/document/4483460/?tp=&arnumber=4483460&isnumber=4483444},
doi={http://10.1109/TDEI.2008.4483460},
abstract={The influence of humidity on streamer propagation at conditions from the threshold for propagation to those for streamer-induced breakdown was investigated in a uniform field in air at atmospheric pressure. Experiments were carried out in a three electrode arrangement consisting of a 12 cm long parallel-plane gap, with an auxiliary needle in the earthed anode. Positive streamers were initiated by applying at the needle electrode a pulse voltage which varied in amplitude. These propagated towards the upper plane electrode which was stressed by a negative dc voltage. Under natural atmospheric conditions, propagation and breakdown probability curves were obtained for several values of absolute humidity in the range between 5 and 22 g/m3. Thus, distributions of the electric field required for streamer propagation and breakdown were obtained and the associated velocity of propagation and time to breakdown were measured. Besides humidity, the amplitude of the voltage used for streamer initiation and the ambient electric field were considered as influencing parameters on streamer properties. Empirical equations are presented expressing the effects of the above parameters on the intrinsic streamer properties. A comparison with previous work in the literature is made and this leads to the conclusion that the influence of humidity on streamer propagation and breakdown can be placed in a sounder quantitative basis.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
IEEE Trans. on Power Delivery, 23, (3), pp. 1571-1580, 2008 Jul
[Abstract][BibTex][pdf]

The relation between the striking distance and interception probability is experimentally investigated under standard lightning impulse voltages of both polarities. By using the rod-plane gap as a reference case, results are presented relating to the discharge interception probability of an earthed rod inserted in the gap. There is a critical separation distance between the stressed and earthed rods at which breakdown occurs either to the plane or to the earthed rod with the same probability. Besides the impulse voltage polarity, the length of the reference gap and the earthed rod height are considered as parameters influencing on interception probability. Results are discussed on the basis of the breakdown mechanism and electric field calculations. The striking distance is defined with the aid of an expression that takes into account, besides lightning peak current, the interception probability and the effects of lightning polarity and struck object height. The results introduce the design of lightning protection systems on the basis of lightning interception probability.

@article{Mikropoulos2008TPD,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Striking distance and interception probability},
journal={IEEE Trans. on Power Delivery},
volume={23},
number={3},
pages={1571-1580},
year={2008},
month={07},
date={2008-07-24},
url={http://ieeexplore.ieee.org/document/4544569/?arnumber=4544569},
doi={http://%2010.1109/TPWRD.2007.916172},
abstract={The relation between the striking distance and interception probability is experimentally investigated under standard lightning impulse voltages of both polarities. By using the rod-plane gap as a reference case, results are presented relating to the discharge interception probability of an earthed rod inserted in the gap. There is a critical separation distance between the stressed and earthed rods at which breakdown occurs either to the plane or to the earthed rod with the same probability. Besides the impulse voltage polarity, the length of the reference gap and the earthed rod height are considered as parameters influencing on interception probability. Results are discussed on the basis of the breakdown mechanism and electric field calculations. The striking distance is defined with the aid of an expression that takes into account, besides lightning peak current, the interception probability and the effects of lightning polarity and struck object height. The results introduce the design of lightning protection systems on the basis of lightning interception probability.}
}

2008

Conference Papers

Petros Mavroidis Pantelis N. Mikropoulos Constantine A. Stassinopoulos G. Smaragdakis
2008 43rd International Universities Power Engineering Conference, pp. 1-5, 2008 Sep
[Abstract][BibTex][pdf]

Results are presented concerning breakdown of short rod-plane air gaps with a dielectric-covered rod under positive standard lightning and switching impulse voltages. Breakdown probability distributions were obtained through multiple level tests and the time and voltage to breakdown were measured with the gap length as a parameter. The simple air gaps with a bare rod were considered as reference cases. There is an increase in the dielectric strength of the gap due to the insulating cover, especially under switching impulse voltages. In the presence of the dielectric cover the effect of the impulse waveform on the dielectric strength of the gap is minimal.

@conference{4651623,
author={Petros Mavroidis Pantelis N. Mikropoulos Constantine A. Stassinopoulos G. Smaragdakis},
title={Impulse breakdown of short rod-plane air gaps with a dielectric covered rod},
booktitle={2008 43rd International Universities Power Engineering Conference},
pages={1-5},
year={2008},
month={09},
date={2008-09-01},
url={http://ieeexplore.ieee.org/document/4651623/},
doi={http://10.1109/UPEC.2008.4651623},
abstract={Results are presented concerning breakdown of short rod-plane air gaps with a dielectric-covered rod under positive standard lightning and switching impulse voltages. Breakdown probability distributions were obtained through multiple level tests and the time and voltage to breakdown were measured with the gap length as a parameter. The simple air gaps with a bare rod were considered as reference cases. There is an increase in the dielectric strength of the gap due to the insulating cover, especially under switching impulse voltages. In the presence of the dielectric cover the effect of the impulse waveform on the dielectric strength of the gap is minimal.}
}

Petros Mavroidis, Pantelis N. Mikropoulos, Constantine A. Stassinopoulos, A Dodos and P Zannias
XVII Int. Conference on Gas Discharges and their Applications, pp. 289-292, Cardiff, UK, 2008 Sep
[Abstract][BibTex][pdf]

Results are presented concerning both prebreakdown phenomena and the breakdown mechanism in short rod-plane gaps with dielectric-covered rod under positive standard lightning impulse voltages. Characteristic discharge parameters, namely first and second corona inception time and voltage, and time and voltage to breakdown were measured, through multiple level tests, with the gap length as a parameter. The simple air gaps i.e. without any insulating covers were considered as reference cases. In the presence of the dielectric cover breakdown occurs at higher applied voltages as a result of increased both corona inception voltages and discharge path. Interpretation of the results is made based on the discharge development pattern and electric field calculations.

@conference{5379232,
author={Petros Mavroidis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos and A Dodos and P Zannias},
title={Discharge characteristics in short rod-plane gaps with dielectric-covered rod under lightning impulse voltages},
booktitle={XVII Int. Conference on Gas Discharges and their Applications},
pages={289-292},
address={Cardiff, UK},
year={2008},
month={09},
date={2008-09-07},
url={http://ieeexplore.ieee.org/document/5379232/?tp=&arnumber=5379232},
abstract={Results are presented concerning both prebreakdown phenomena and the breakdown mechanism in short rod-plane gaps with dielectric-covered rod under positive standard lightning impulse voltages. Characteristic discharge parameters, namely first and second corona inception time and voltage, and time and voltage to breakdown were measured, through multiple level tests, with the gap length as a parameter. The simple air gaps i.e. without any insulating covers were considered as reference cases. In the presence of the dielectric cover breakdown occurs at higher applied voltages as a result of increased both corona inception voltages and discharge path. Interpretation of the results is made based on the discharge development pattern and electric field calculations.}
}

Lazaros Lazaridis Pantelis N. Mikropoulos A. Darras A. Theocharis
2008 17th International Conference on Gas Discharges and Their Applications, pp. 233-236, 2008 Sep
[Abstract][BibTex][pdf]

Results are presented concerning discharge development and breakdown along cylindrical insulating surfaces in a 12 cm long rod-plane gap under positive standard lightning impulse voltages. Cylindrical insulator specimens, made of silicone rubber and glazed porcelain, could be inserted parallel to the rod electrode, bridging the gap; the case of air alone was regarded as reference. Discharge development patterns were obtained by oscillographic monitoring of the electric field strength at the earthed plane. An insulating surface, depending on material permittivity, may inhibit the development of the initial corona streamers but favour breakdown.

@conference{5379263,
author={Lazaros Lazaridis Pantelis N. Mikropoulos A. Darras A. Theocharis},
title={Flashover along cylindrical insulating surfaces under positive lightning impulse voltages},
booktitle={2008 17th International Conference on Gas Discharges and Their Applications},
pages={233-236},
year={2008},
month={09},
date={2008-09-01},
url={http://ieeexplore.ieee.org/document/5379263/},
abstract={Results are presented concerning discharge development and breakdown along cylindrical insulating surfaces in a 12 cm long rod-plane gap under positive standard lightning impulse voltages. Cylindrical insulator specimens, made of silicone rubber and glazed porcelain, could be inserted parallel to the rod electrode, bridging the gap; the case of air alone was regarded as reference. Discharge development patterns were obtained by oscillographic monitoring of the electric field strength at the earthed plane. An insulating surface, depending on material permittivity, may inhibit the development of the initial corona streamers but favour breakdown.}
}

Pantelis N. Mikropoulos, Thomas Tsovilis, P Chatzidimitriou and P Vasilaras
MedPower 2008, Thessaloniki, Greece, 2008 Sep
[Abstract][BibTex][pdf]

A user-friendly Windows application software has been developed for shielding design of high voltage substations against direct lightning strokes; shielding design can be achieved in a few minutes on the basis of a 3-dimensional analysis. With the aid of the software, an installed shielding system can be validated and/or a new system can be designed according to IEEE Standard 998:1996. The performance of different shielding design methods can be easily evaluated for various operating system voltages and equipment dimensions. The developed software has been applied to the shielding design of typical substations of the Hellenic Transmission System, 150 kV and 400 kV substations and a comparison of the design methods has been made. The application software is a useful tool for electrical engineers and can also be used for educational purposes in high voltage engineering courses.

@conference{Mikropoulos2008MP,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and P Chatzidimitriou and P Vasilaras},
title={Software development for direct lightning stroke shielding of substations},
booktitle={MedPower 2008},
address={Thessaloniki, Greece},
year={2008},
month={09},
date={2008-09-01},
url={https://www.researchgate.net/publication/260999806_Software_Development_for_Direct_Lightning_Stroke_Shielding_of_Substations},
abstract={A user-friendly Windows application software has been developed for shielding design of high voltage substations against direct lightning strokes; shielding design can be achieved in a few minutes on the basis of a 3-dimensional analysis. With the aid of the software, an installed shielding system can be validated and/or a new system can be designed according to IEEE Standard 998:1996. The performance of different shielding design methods can be easily evaluated for various operating system voltages and equipment dimensions. The developed software has been applied to the shielding design of typical substations of the Hellenic Transmission System, 150 kV and 400 kV substations and a comparison of the design methods has been made. The application software is a useful tool for electrical engineers and can also be used for educational purposes in high voltage engineering courses.}
}

Pantelis N. Mikropoulos, Thomas Tsovilis and T Ananiadis
MedPower 2008, Thessaloniki, Greece, 2008 Sep
[Abstract][BibTex][pdf]

The effect of an earthed object on the interception radius of the Franklin rod is experimentally investigated under standard lightning impulse voltages of both polarities. By using a 75 cm rod-plane gap as a reference case, results are presented relating to the discharge interception probability of an earthed rod inserted in the gap together with a neighbouring shorter earthed rod. At certain separation distances between stressed and earthed rods breakdown occurs to either of the earthed electrodes, plane and two rods, with the same probability (33.3%); these critical separation distances depend on impulse volt¬age polarity and earthed rods height. The experimental results are interpreted in terms of the breakdown mechanism and their possible extension to shielding design is discussed. The interception probability dis-tribution of a Franklin rod is affected by the presence of a neighbouring earthed object.

@conference{Mikropoulos2008MPTsovilis,
author={Pantelis N. Mikropoulos and Thomas Tsovilis and T Ananiadis},
title={The effect of an earthed object on the interception radius of the Franklin rod: An experimental investigation},
booktitle={MedPower 2008},
address={Thessaloniki, Greece},
year={2008},
month={09},
date={2008-09-01},
url={https://www.researchgate.net/publication/235986251_The_effect_of_an_earthed_object_on_the_interception_radius_of_the_Franklin_rod_An_experimental_investigation},
abstract={The effect of an earthed object on the interception radius of the Franklin rod is experimentally investigated under standard lightning impulse voltages of both polarities. By using a 75 cm rod-plane gap as a reference case, results are presented relating to the discharge interception probability of an earthed rod inserted in the gap together with a neighbouring shorter earthed rod. At certain separation distances between stressed and earthed rods breakdown occurs to either of the earthed electrodes, plane and two rods, with the same probability (33.3%); these critical separation distances depend on impulse volt¬age polarity and earthed rods height. The experimental results are interpreted in terms of the breakdown mechanism and their possible extension to shielding design is discussed. The interception probability dis-tribution of a Franklin rod is affected by the presence of a neighbouring earthed object.}
}

2007

Conference Papers

Lazaros Lazaridis, Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
"Breakdown in air and along a porcelain insulator under positive switching impulse voltages"
15th International Symposium on High Voltage Engineering, Ljubljana, 2007 Aug
[Abstract][BibTex]

@conference{Lazaridis2007,
author={Lazaros Lazaridis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Breakdown in air and along a porcelain insulator under positive switching impulse voltages},
booktitle={15th International Symposium on High Voltage Engineering},
address={Ljubljana},
year={2007},
month={08},
date={2007-08-01}
}

Petros Mavroidis, Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
42nd Universities Power Engineering Conference, Brighton, UK, 2007 Sep
[Abstract][BibTex][pdf]

Results are presented concerning prebreakdown phenomena and breakdown in short rod-plane gaps in air under lightning impulse voltages of both polarities. Breakdown probability distributions were obtained and the discharge parameters such as first and second corona inception time and voltage, dark period, time and voltage required for the discharge to arrive at the earthed plane and for breakdown were measured as a function of gap length and impulse voltage polarity. Breakdown under negative impulse voltages is closely associated with the prebreakdown phenomena since there is a close correlation between coronas inception and breakdown voltages; this is not always the case under positive impulse voltages. For both polarities there is a tight correlation between the breakdown and discharge arrival voltages. At 50% breakdown probability, the mean electric field required for the discharge to arrive at the plane is roughly two times higher under negative than positive impulse voltages and for both polarities it decreases with gap length according to a power law. These explain the similar dependence of the 50% breakdown voltage on polarity and gap length.

@conference{Mavroidis2007,
author={Petros Mavroidis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Discharge characteristics in short rod-plane gaps under lightning impulse voltages of both polarities},
booktitle={42nd Universities Power Engineering Conference},
address={Brighton, UK},
year={2007},
month={09},
date={2007-09-04},
url={http://ieeexplore.ieee.org/document/4469099/},
doi={http://10.1109/UPEC.2007.4469099},
abstract={Results are presented concerning prebreakdown phenomena and breakdown in short rod-plane gaps in air under lightning impulse voltages of both polarities. Breakdown probability distributions were obtained and the discharge parameters such as first and second corona inception time and voltage, dark period, time and voltage required for the discharge to arrive at the earthed plane and for breakdown were measured as a function of gap length and impulse voltage polarity. Breakdown under negative impulse voltages is closely associated with the prebreakdown phenomena since there is a close correlation between coronas inception and breakdown voltages; this is not always the case under positive impulse voltages. For both polarities there is a tight correlation between the breakdown and discharge arrival voltages. At 50% breakdown probability, the mean electric field required for the discharge to arrive at the plane is roughly two times higher under negative than positive impulse voltages and for both polarities it decreases with gap length according to a power law. These explain the similar dependence of the 50% breakdown voltage on polarity and gap length.}
}

Pantelis N. Mikropoulos and Thomas Tsovilis
15th International Symposium on High Voltage Engineering, Ljubljana, 2007 Aug
[Abstract][BibTex][pdf]

The protection zone of the Franklin rod is experimentally investigated under standard lightning impulse voltages of both polarities. By using a 75 cm rod-plane gap as a reference case, results are presented relating to the discharge interception probability of an earthed rod inserted in the gap. There is a critical lateral distance between the stressed and earthed rods at which breakdown occurs either to the plane or to the earthed rod with the same probability; this depends on impulse volt¬age polarity and earthed rod height. The Franklin rod protection zone is de¬signed at different interception probabilities on the basis of an expression defining its boundaries. The protection zone identified according to the rolling sphere method is overesti¬mated for positive lightning but underestimated in the case of negative lightning. An explanation of this behaviour is attempted relating to the breakdown mechanism which is dis¬cussed with the aid of electric field calculations.

@conference{Mikropoulos2007ISHVE,
author={Pantelis N. Mikropoulos and Thomas Tsovilis},
title={Experimental investigation of the Franklin rod protection zone},
booktitle={15th International Symposium on High Voltage Engineering},
address={Ljubljana},
year={2007},
month={08},
date={2007-08-01},
url={https://www.researchgate.net/publication/235986302_Experimental_investigation_of_the_Franklin_rod_protection_zone},
abstract={The protection zone of the Franklin rod is experimentally investigated under standard lightning impulse voltages of both polarities. By using a 75 cm rod-plane gap as a reference case, results are presented relating to the discharge interception probability of an earthed rod inserted in the gap. There is a critical lateral distance between the stressed and earthed rods at which breakdown occurs either to the plane or to the earthed rod with the same probability; this depends on impulse volt¬age polarity and earthed rod height. The Franklin rod protection zone is de¬signed at different interception probabilities on the basis of an expression defining its boundaries. The protection zone identified according to the rolling sphere method is overesti¬mated for positive lightning but underestimated in the case of negative lightning. An explanation of this behaviour is attempted relating to the breakdown mechanism which is dis¬cussed with the aid of electric field calculations.}
}

2006

Conference Papers

Lazaros Lazaridis, Pantelis N. Mikropoulos, Constantine A. Stassinopoulos, C Kerasaridis and I Lisaridis
41st Universities Power Engineering Conference, pp. 912-915, Newcastle, UK, 2006 Sep
[Abstract][BibTex][pdf]

Results are presented concerning corona inception in air and in the presence of a porcelain insulator in a non-uniform field under positive switching impulse voltages. It is shown that in the presence of the insulator both first and second coronas start earlier, under lower inception voltages and with smaller associated charges compared to corona inception in air alone. Where the insulator is present, the first and second corona inception voltages and the required voltage increase during dark period increase linearly with the applied voltage. This is not so in the case of corona inception in air alone where the second corona inception voltage was found roughly constant, independent of the applied voltage variations. The apparent first and second corona charges were found to increase with the corresponding inception voltages according to a power and an exponential law respectively

@conference{Lazaridis2006,
author={Lazaros Lazaridis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos and C Kerasaridis and I Lisaridis},
title={Corona inception in the presence of insulators in a rod-plane gap under positive switching impulse voltages},
booktitle={41st Universities Power Engineering Conference},
pages={912-915},
address={Newcastle, UK},
year={2006},
month={09},
date={2006-09-06},
url={http://ieeexplore.ieee.org/document/4218820/},
doi={http://10.1109/UPEC.2006.367612},
abstract={Results are presented concerning corona inception in air and in the presence of a porcelain insulator in a non-uniform field under positive switching impulse voltages. It is shown that in the presence of the insulator both first and second coronas start earlier, under lower inception voltages and with smaller associated charges compared to corona inception in air alone. Where the insulator is present, the first and second corona inception voltages and the required voltage increase during dark period increase linearly with the applied voltage. This is not so in the case of corona inception in air alone where the second corona inception voltage was found roughly constant, independent of the applied voltage variations. The apparent first and second corona charges were found to increase with the corresponding inception voltages according to a power and an exponential law respectively}
}

Pantelis N. Mikropoulos, B.C Sarigiannidou, M Stapountzi and Constantine A. Stassinopoulos
41st Universities Power Engineering Conference, pp. 916-920, Newcastle, UK, 2006 Sep
[Abstract][BibTex][pdf]

Results are presented concerning the influence of humidity on streamer propagation and flashover along a PTFE cylindrical insulator in a uniform field in air. Experiments were carried out in a three electrode arrangement consisting of a parallel-plane gap (12 cm), stressed by negative DC voltage and a needle in the earthed anode. Positive streamers were initiated by applying a pulse voltage at the needle electrode and propagate along the insulator surface. Streamer properties investigated are the electric field required for stable streamer propagation and the propagation velocity with field values sustaining stable streamer propagation and also under \"overfields\". The electric field required for flashover and the corresponding time were measured. It is shown that the electric field required for stable streamer propagation and the associated velocity increase with increasing humidity, whilst under \"overfields\" high humidity causes streamers to decelerate. Time to breakdown tends to decrease while the required field to breakdown increases with increasing humidity

@conference{Mikropoulos2006UPEC,
author={Pantelis N. Mikropoulos and B.C Sarigiannidou and M Stapountzi and Constantine A. Stassinopoulos},
title={Streamer propagation and flashover along insulator surface in a uniform field in air: influence of humidity},
booktitle={41st Universities Power Engineering Conference},
pages={916-920},
address={Newcastle, UK},
year={2006},
month={09},
date={2006-09-06},
url={http://ieeexplore.ieee.org/document/4218821/},
doi={http://10.1109/UPEC.2006.367613},
abstract={Results are presented concerning the influence of humidity on streamer propagation and flashover along a PTFE cylindrical insulator in a uniform field in air. Experiments were carried out in a three electrode arrangement consisting of a parallel-plane gap (12 cm), stressed by negative DC voltage and a needle in the earthed anode. Positive streamers were initiated by applying a pulse voltage at the needle electrode and propagate along the insulator surface. Streamer properties investigated are the electric field required for stable streamer propagation and the propagation velocity with field values sustaining stable streamer propagation and also under \\"overfields\\". The electric field required for flashover and the corresponding time were measured. It is shown that the electric field required for stable streamer propagation and the associated velocity increase with increasing humidity, whilst under \\"overfields\\" high humidity causes streamers to decelerate. Time to breakdown tends to decrease while the required field to breakdown increases with increasing humidity}
}

2005

Journal Articles

D.E. Gourgoulis, Pantelis N. Mikropoulos, Constantine A. Stassinopoulos and C.G. Yakinthos
IEE Proceedings - Science, Measurement and Technology, 152, (4), pp. 155-160, 2005 Jul
[Abstract][BibTex][pdf]

The positive impulse breakdown behaviour of conductor-rod gaps 50?cm in length is investigated while negative DC voltages pre-stress the gap. Breakdown voltage and time to breakdown variations are discussed in terms of the parameters influencing the discharge mechanism such as the gap geometry, the amplitude of the pre-stress negative DC voltage applied at the rod, the waveshape of the applied impulse voltage at the conductor and the variation of absolute humidity. It is shown that pre-stressing, by producing sufficient DC coronas at both electrodes, results in a significant increase in the breakdown voltage. There is a threshold value of the pre-stress DC voltage above which the gap factor against breakdown may double its value. Both absolute humidity and pre-stressing affect, in a combined way, the dielectric strength of the gaps.

@article{Gourgoulis205,
author={D.E. Gourgoulis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos and C.G. Yakinthos},
title={Effects of negative DC pre-stressing on positive impulse breakdown characteristics of conductor-rod gaps},
journal={IEE Proceedings - Science, Measurement and Technology},
volume={152},
number={4},
pages={155-160},
year={2005},
month={07},
date={2005-07-01},
url={http://digital-library.theiet.org/content/journals/10.1049/ip-smt_20045006},
abstract={The positive impulse breakdown behaviour of conductor-rod gaps 50?cm in length is investigated while negative DC voltages pre-stress the gap. Breakdown voltage and time to breakdown variations are discussed in terms of the parameters influencing the discharge mechanism such as the gap geometry, the amplitude of the pre-stress negative DC voltage applied at the rod, the waveshape of the applied impulse voltage at the conductor and the variation of absolute humidity. It is shown that pre-stressing, by producing sufficient DC coronas at both electrodes, results in a significant increase in the breakdown voltage. There is a threshold value of the pre-stress DC voltage above which the gap factor against breakdown may double its value. Both absolute humidity and pre-stressing affect, in a combined way, the dielectric strength of the gaps.}
}

Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
IEE Proc.-Sci. Meas. Technol., 152, (4), pp. 169-174, 2005 Jul
[Abstract][BibTex][pdf]

The sparkover behaviour of sphere-rod gaps under standard impulse voltages is investigated by assessing the influence of gap spacing, impulse voltage waveshape and polarity, and atmospheric conditions on the distribution of the sparkover voltage. At sparkover the growth of streamers of both polarities can be estimated since a value of 2 MV/m is found to be characteristic for the average negative streamer gradient. The IEC atmospheric conditions correction is discussed and compared with a new optimised correction procedure. The feasibility of using the sphere-rod gap as substandard for measuring impulse voltages, replacing the conventional more expensive sphere gap, is examined.

@article{Mikropoulos2005,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Impulse sparkover characteristics of sphere-rod gaps},
journal={IEE Proc.-Sci. Meas. Technol.},
volume={152},
number={4},
pages={169-174},
year={2005},
month={07},
date={2005-07-01},
url={http://digital-library.theiet.org/content/journals/10.1049/ip-smt_20045030},
issn={1350-2344},
abstract={The sparkover behaviour of sphere-rod gaps under standard impulse voltages is investigated by assessing the influence of gap spacing, impulse voltage waveshape and polarity, and atmospheric conditions on the distribution of the sparkover voltage. At sparkover the growth of streamers of both polarities can be estimated since a value of 2 MV/m is found to be characteristic for the average negative streamer gradient. The IEC atmospheric conditions correction is discussed and compared with a new optimised correction procedure. The feasibility of using the sphere-rod gap as substandard for measuring impulse voltages, replacing the conventional more expensive sphere gap, is examined.}
}

2005

Conference Papers

Pantelis N. Mikropoulos, Constantine A. Stassinopoulos and Charalambos Yakinthos
"Αύξηση της δυνατότητας μεταφοράς ενέργειας σε υψηλές τάσεις μέσω της συμπίεσης των διακένων"
1o Πανελλήνιο Συνέδριο Διπλ. Μηχανολόγων – Ηλεκτρολόγων, Αθήνα, 2005 Jan
[Abstract][BibTex]

@conference{Mikropoulos2005AthensPan,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos and Charalambos Yakinthos},
title={Αύξηση της δυνατότητας μεταφοράς ενέργειας σε υψηλές τάσεις μέσω της συμπίεσης των διακένων},
booktitle={1o Πανελλήνιο Συνέδριο Διπλ. Μηχανολόγων – Ηλεκτρολόγων},
address={Αθήνα},
year={2005},
month={01},
date={2005-01-01}
}

Pantelis N. Mikropoulos, B.C. Sarigiannidou, Constantine A. Stassinopoulos and C Tsakiridis
40th Universities Power Engineering Conference, pp. 803-807, Cork, 2005 Jan
[Abstract][BibTex][pdf]

Results are presented concerning the influence of humidity on streamer propagation and breakdown in a uniform field in air at atmospheric pressure with atmospheric conditions varying naturally. Experiments were carried out in a three electrode arrangement consisting of a parallel-plane gap, 12 cm spacing, and a needle in the earthed anode. Positive streamers are initiated by applying a pulse voltage, varying in amplitude, at the needle electrode and propagate towards the upper plane electrode which is stressed by negative DC voltage. Streamer properties investigated are the electric field required for stable streamer propagation and the propagation velocity under field values sustaining stable streamer propagation and under overfields. Time to breakdown and the corresponding field are also measured. Propagation and breakdown probability curves are obtained for several values of absolute humidity in the range between 5.5 and 18 g/m3. It is shown that increasing humidity results in higher field required for streamer propagation and breakdown and in greater propagation velocity. Time to breakdown decreases with increasing humidity.

@conference{Mikropoulos2005UPEC,
author={Pantelis N. Mikropoulos and B.C. Sarigiannidou and Constantine A. Stassinopoulos and C Tsakiridis},
title={Influence of humidity on positive streamer propagation and breakdown in a uniform field in air},
booktitle={40th Universities Power Engineering Conference},
pages={803-807},
address={Cork},
year={2005},
month={01},
date={2005-01-01},
url={https://www.researchgate.net/publication/255948736_Influence_of_humidity_on_positive_streamer_propagation_and_breakdown_in_a_uniform_field_in_air},
abstract={Results are presented concerning the influence of humidity on streamer propagation and breakdown in a uniform field in air at atmospheric pressure with atmospheric conditions varying naturally. Experiments were carried out in a three electrode arrangement consisting of a parallel-plane gap, 12 cm spacing, and a needle in the earthed anode. Positive streamers are initiated by applying a pulse voltage, varying in amplitude, at the needle electrode and propagate towards the upper plane electrode which is stressed by negative DC voltage. Streamer properties investigated are the electric field required for stable streamer propagation and the propagation velocity under field values sustaining stable streamer propagation and under overfields. Time to breakdown and the corresponding field are also measured. Propagation and breakdown probability curves are obtained for several values of absolute humidity in the range between 5.5 and 18 g/m3. It is shown that increasing humidity results in higher field required for streamer propagation and breakdown and in greater propagation velocity. Time to breakdown decreases with increasing humidity.}
}

G D. Sergiadis, D S. Aleksiadis and Constantine A. Stassinopoulos
"Η αρχή της συνετής αποφυγής – Μια πρόταση αντιμετώπισης των πιθανών κινδύνων από την έκθεση σε πεδία γραμμών ΥΤ"
1o Πανελλήνιο Συνέδριο Διπλ. Μηχανολόγων – Ηλεκτρολόγων, Αθήνα, 2005 Jan
[Abstract][BibTex]

@conference{Sergiadis2005Athens,
author={G D. Sergiadis and D S. Aleksiadis and Constantine A. Stassinopoulos},
title={Η αρχή της συνετής αποφυγής – Μια πρόταση αντιμετώπισης των πιθανών κινδύνων από την έκθεση σε πεδία γραμμών ΥΤ},
booktitle={1o Πανελλήνιο Συνέδριο Διπλ. Μηχανολόγων – Ηλεκτρολόγων},
address={Αθήνα},
year={2005},
month={01},
date={2005-01-01}
}

2004

Conference Papers

Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
39th Universities Power Engineering Conference, pp. 207-210, Bristol, UK, 2004 Sep
[Abstract][BibTex][pdf]

The influence of atmospheric conditions on the dielectric strength of external insulation is a complicated matter. Investigations carried out resulted in a correction procedure for atmospheric conditions that is currently adopted by the IEC standard 60060-1/1989. Despite its widespread applicability, many of the factors employed in this standard are still under consideration and several authors have argued about its validity, especially on short air gaps. The paper presents data concerning the breakdown characteristics of rod-plane gaps under positive impulse voltages. Emphasis is given on the influence of atmospheric conditions, the effect of gap spacing and the wavefront duration of the applied impulse voltages. Empirical expressions relating to these parameters are presented. An optimised atmospheric correction procedure is introduced, which employs an iterative concurrent correction for both air density and humidity. It estimates the influence of atmospheric conditions more accurately than the IEC procedure.

@conference{Mikropoulos2004,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Atmospheric correction in rod-plane gaps up to 1 m in length},
booktitle={39th Universities Power Engineering Conference},
pages={207-210},
address={Bristol, UK},
year={2004},
month={09},
date={2004-09-06},
url={http://ieeexplore.ieee.org/document/1491993/},
abstract={The influence of atmospheric conditions on the dielectric strength of external insulation is a complicated matter. Investigations carried out resulted in a correction procedure for atmospheric conditions that is currently adopted by the IEC standard 60060-1/1989. Despite its widespread applicability, many of the factors employed in this standard are still under consideration and several authors have argued about its validity, especially on short air gaps. The paper presents data concerning the breakdown characteristics of rod-plane gaps under positive impulse voltages. Emphasis is given on the influence of atmospheric conditions, the effect of gap spacing and the wavefront duration of the applied impulse voltages. Empirical expressions relating to these parameters are presented. An optimised atmospheric correction procedure is introduced, which employs an iterative concurrent correction for both air density and humidity. It estimates the influence of atmospheric conditions more accurately than the IEC procedure.}
}

Pantelis N. Mikropoulos, Constantine A. Stassinopoulos, I Katarachias and A Tsetoglou
"Sparkover characteristics of sphere-rod gaps under standard impulse voltages"
WSEAS Transactions on Circuits and Systems, pp. 1197-1200, 2004 Jan
[Abstract][BibTex]

@conference{Mikropoulos2004WSEAS,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos and I Katarachias and A Tsetoglou},
title={Sparkover characteristics of sphere-rod gaps under standard impulse voltages},
booktitle={WSEAS Transactions on Circuits and Systems},
pages={1197-1200},
year={2004},
month={01},
date={2004-01-01}
}

Pantelis N. Mikropoulos, Constantine A. Stassinopoulos and Charalambos Yakinthos
X International Symposium on Gaseous Dielectrics, pp. 229-234, Athens, Greece, 2004 Jan
[Abstract][BibTex][pdf]

The influence of the superimposition of positive impulses on negative DC pre-stressed conductor-rod gaps with spacing of 50 cm is investigated. Previous experiments have shown that when a conductor of 2 cm in diameter is used a marked increase in the dielectric strength of the gap is observed when negative DC voltages between 100 kV and 125 kV in value are applied at the rod; this is not the case for a 3 cm conductor. This paper provides new data concerning the above phenomenon; by applying higher values of pre-stressing voltage a similar behaviour as with the 2 cm conductor is observed for the 3 cm conductor. The discharge mechanism is discussed taking into consideration parameters affecting the above behaviour, such as the value of the pre-stressing DC voltage, the waveshape of the applied impulse voltage and the variation of absolute humidity.

@conference{Mikropoulos2004XISGD,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos and Charalambos Yakinthos},
title={Negative DC pre-stressing on conductor-rod gaps under positive impulse voltages},
booktitle={X International Symposium on Gaseous Dielectrics},
pages={229-234},
address={Athens, Greece},
year={2004},
month={01},
date={2004-01-01},
url={https://link.springer.com/chapter/10.1007/978-1-4419-8979-6_32},
abstract={The influence of the superimposition of positive impulses on negative DC pre-stressed conductor-rod gaps with spacing of 50 cm is investigated. Previous experiments have shown that when a conductor of 2 cm in diameter is used a marked increase in the dielectric strength of the gap is observed when negative DC voltages between 100 kV and 125 kV in value are applied at the rod; this is not the case for a 3 cm conductor. This paper provides new data concerning the above phenomenon; by applying higher values of pre-stressing voltage a similar behaviour as with the 2 cm conductor is observed for the 3 cm conductor. The discharge mechanism is discussed taking into consideration parameters affecting the above behaviour, such as the value of the pre-stressing DC voltage, the waveshape of the applied impulse voltage and the variation of absolute humidity.}
}

2003

Conference Papers

D E. Gourgoulis, Constantine A. Stassinopoulos, G D. Tonozlis, Charalambos Yakinthos and S Chondrogiannis
"A comparative presentation of sphere gaps and sphere-rod gaps under standard lightning impulse voltages"
13th International Symposium in High Voltage Engineering, Netherlands, 2003 Jan
[Abstract][BibTex]

@conference{Gourgoulis2003,
author={D E. Gourgoulis and Constantine A. Stassinopoulos and G D. Tonozlis and Charalambos Yakinthos and S Chondrogiannis},
title={A comparative presentation of sphere gaps and sphere-rod gaps under standard lightning impulse voltages},
booktitle={13th International Symposium in High Voltage Engineering},
address={Netherlands},
year={2003},
month={01},
date={2003-01-01}
}

G Sergiadis and Constantine A. Stassinopoulos
"Consequences of overhead lines on public health"
38th Universities Power Engineering Conference, pp. 225, Thessaloniki, Greece, 2003 Jan
[Abstract][BibTex]

@conference{Sergiadis2003,
author={G Sergiadis and Constantine A. Stassinopoulos},
title={Consequences of overhead lines on public health},
booktitle={38th Universities Power Engineering Conference},
pages={225},
address={Thessaloniki, Greece},
year={2003},
month={01},
date={2003-01-01}
}

2002

Conference Papers

Constantine A. Stassinopoulos, G D. Tonozlis and Charalambos Yakinthos
"Comparison between combined direct and impulse voltages and D.C. voltages stressed tower-line clearance"
37th Universities Power Engineering Conference, pp. 401, Stafford, 2002 Jan
[Abstract][BibTex]

@conference{Stassinopoulos2002,
author={Constantine A. Stassinopoulos and G D. Tonozlis and Charalambos Yakinthos},
title={Comparison between combined direct and impulse voltages and D.C. voltages stressed tower-line clearance},
booktitle={37th Universities Power Engineering Conference},
pages={401},
address={Stafford},
year={2002},
month={01},
date={2002-01-01}
}

2001

Journal Articles

N. L. Allen and Pantelis N. Mikropoulos
IEEE Transactions on Dielectrics and Electrical Insulation, 8, (5), pp. 812-817, 2001 Oct
[Abstract][BibTex][pdf]

In a uniform field arrangement, under direct voltage, positive streamer propagation and breakdown are investigated along cylindrical insulators with different profiles, inserted perpendicularly between two parallel plane electrodes. The basic properties of streamer propagation and breakdown, namely the electric field required for a stable propagation together with the associated velocity and the breakdown field together with time to breakdown, are measured as influenced by the pulse voltage amplitude used for the streamer initiation and by the insulator profile. It is shown that a strong relation between streamer propagation and breakdown exists, because the insulator profile exerts a similar influence on the breakdown and propagation fields. The effect of a shed on an insulating surface, forming an `obstruction\' to streamer progress, is to increase the stability for propagation and breakdown fields, and to reduce the propagation velocity at all applied fields compared with those in the case of a smooth insulator. Along the surface of a smooth insulator, a streamer system propagates with a `surface\' and an `air\' component; however, a shed on an insulating surface modifies this system, resulting in only one component reaching the cathode

@article{Allen2001,
author={N. L. Allen and Pantelis N. Mikropoulos},
title={Surface profile effect on streamer propagation and breakdown in air},
journal={IEEE Transactions on Dielectrics and Electrical Insulation},
volume={8},
number={5},
pages={812-817},
year={2001},
month={10},
date={2001-10-01},
url={http://ieeexplore.ieee.org/document/959708/?arnumber=959708},
doi={http://10.1109/94.959708},
keywords={air;electric breakdown;surface discharges;cylindrical insulator;electric field;parallel plane electrodes;shed;streamer propagation;surface profile;Breakdown voltage;Cathodes;Dielectrics and electrical insulation;Electric variables measurement;Electrodes;Pulse measurements;Stability;Time measurement;Velocity measurement},
abstract={In a uniform field arrangement, under direct voltage, positive streamer propagation and breakdown are investigated along cylindrical insulators with different profiles, inserted perpendicularly between two parallel plane electrodes. The basic properties of streamer propagation and breakdown, namely the electric field required for a stable propagation together with the associated velocity and the breakdown field together with time to breakdown, are measured as influenced by the pulse voltage amplitude used for the streamer initiation and by the insulator profile. It is shown that a strong relation between streamer propagation and breakdown exists, because the insulator profile exerts a similar influence on the breakdown and propagation fields. The effect of a shed on an insulating surface, forming an `obstruction\\' to streamer progress, is to increase the stability for propagation and breakdown fields, and to reduce the propagation velocity at all applied fields compared with those in the case of a smooth insulator. Along the surface of a smooth insulator, a streamer system propagates with a `surface\\' and an `air\\' component; however, a shed on an insulating surface modifies this system, resulting in only one component reaching the cathode}
}

2001

Conference Papers

D E. Gourgoulis, Constantine A. Stassinopoulos and Charalambos Yakinthos
36th Universities Power Engineering Conference, Swansea, 2001 Jan
[Abstract][BibTex][pdf]

In overhead lines it is desirable to find the minimum practicable clearances to be used when a composite rather than a simple impulse voltge is applied. In our previous work the conductor - rod gap has been studied; this shows a higher gap factor than other gap configurations. As a part of a programme of work on overhead line compaction at the High Voltage Laboratory of the A.U.Th. the possibility of finding other gap configurations displaying high gap factors has been studied. For this reason the impulse breakdown of a conductor - rod gap with insulator strings as an application analogus to some overhead line situations and a conductor - sphere gap as an alternative configuration of the conductor - rod gap when the rod or the sphere is pre - stressed with negative direct voltage have been investigated. It was found that the effect of pre - stressing depends on the diameter of the conductor, on the position of the rod, on the waveshape of the applied impulse voltage and on the value of the direct voltage.

@conference{Gourgoulis2001,
author={D E. Gourgoulis and Constantine A. Stassinopoulos and Charalambos Yakinthos},
title={Combined direct and impulse voltages to conductor-rod gaps with insulator strings and conductor-sphere air gaps},
booktitle={36th Universities Power Engineering Conference},
address={Swansea},
year={2001},
month={01},
date={2001-01-01},
url={https://www.researchgate.net/publication/290951373_Combined_direct_and_impulse_voltages_to_conductor_-_Rod_gaps_with_insulator_strings_and_condutor_-_Sphere_air_gaps},
abstract={In overhead lines it is desirable to find the minimum practicable clearances to be used when a composite rather than a simple impulse voltge is applied. In our previous work the conductor - rod gap has been studied; this shows a higher gap factor than other gap configurations. As a part of a programme of work on overhead line compaction at the High Voltage Laboratory of the A.U.Th. the possibility of finding other gap configurations displaying high gap factors has been studied. For this reason the impulse breakdown of a conductor - rod gap with insulator strings as an application analogus to some overhead line situations and a conductor - sphere gap as an alternative configuration of the conductor - rod gap when the rod or the sphere is pre - stressed with negative direct voltage have been investigated. It was found that the effect of pre - stressing depends on the diameter of the conductor, on the position of the rod, on the waveshape of the applied impulse voltage and on the value of the direct voltage.}
}

Constantine A. Stassinopoulos and Charalambos Yakinthos
IX International Symposium on Gaseous Dielectrics, pp. 535, Maryland, 2001 Jan
[Abstract][BibTex][pdf]

In order to investigate the breakdown mechanism of a conductor — tower gap under transient overvoltages in the laboratory, the simulation of overvoltages is done by the use of standard lightning or switching impulses. If these impulses are superimposed or preceded by a steady direct pre — stressing voltage it has been shown1,5 that this affects the dielectric strength of the configuration used. Previous study1 showed that the influence of the superimposition of positive impulses on a negative DC pre — stressing conductor-rod gap with a spacing of 50 cm is significant in that under certain conditions the overall sparkover voltage tends to increase. Several parameters affect this behaviour, i.e. the value of the pre — stressing DC voltage, the diameter of the energised conductor, the position of the rod with regard to the conductor, the waveshape of the applied impulse and humidity. All this was explained in terms of the various DC and impulse coronas that occur at the conductor and the rod which tend to create a more «uniform» field distribution in the gap. In order to have a better simulation of a tower — line clearance, a construction similar to a tower and a line has been constructed. Since a conductor — rod gap shows a gap factor of about l.610, rods were inserted in the gap and the influence of their length on the breakdown mechanism has been evaluated.

@conference{Stassinopoulos2001,
author={Constantine A. Stassinopoulos and Charalambos Yakinthos},
title={The breakdown of a tower-line clearance under impulse voltages pre-stressed by a negative D.C. voltage},
booktitle={IX International Symposium on Gaseous Dielectrics},
pages={535},
address={Maryland},
year={2001},
month={01},
date={2001-01-01},
url={https://www.researchgate.net/publication/315569339_The_breakdown_of_a_tower_-_line_clearance_under_impulse_voltages_pre_-_stressed_by_a_negative_DC_voltage},
abstract={In order to investigate the breakdown mechanism of a conductor — tower gap under transient overvoltages in the laboratory, the simulation of overvoltages is done by the use of standard lightning or switching impulses. If these impulses are superimposed or preceded by a steady direct pre — stressing voltage it has been shown1,5 that this affects the dielectric strength of the configuration used. Previous study1 showed that the influence of the superimposition of positive impulses on a negative DC pre — stressing conductor-rod gap with a spacing of 50 cm is significant in that under certain conditions the overall sparkover voltage tends to increase. Several parameters affect this behaviour, i.e. the value of the pre — stressing DC voltage, the diameter of the energised conductor, the position of the rod with regard to the conductor, the waveshape of the applied impulse and humidity. All this was explained in terms of the various DC and impulse coronas that occur at the conductor and the rod which tend to create a more «uniform» field distribution in the gap. In order to have a better simulation of a tower — line clearance, a construction similar to a tower and a line has been constructed. Since a conductor — rod gap shows a gap factor of about l.610, rods were inserted in the gap and the influence of their length on the breakdown mechanism has been evaluated.}
}

Constantine A. Stassinopoulos and G Sergiadis
"Επιπτώσεις των εναερίων γραμμών ΥΤ στη Δημόσια Υγεία"
Παγκόσμιο ΣυνέδριοΜηχανολόγων – Ηλεκτρολόγων, pp. 116, Κοζάνης, 2001 Jan
[Abstract][BibTex]

@conference{Stassinopoulos2001Kozanis,
author={Constantine A. Stassinopoulos and G Sergiadis},
title={Επιπτώσεις των εναερίων γραμμών ΥΤ στη Δημόσια Υγεία},
booktitle={Παγκόσμιο ΣυνέδριοΜηχανολόγων – Ηλεκτρολόγων},
pages={116},
address={Κοζάνης},
year={2001},
month={01},
date={2001-01-01}
}

2000

Conference Papers

Z D. Giagridis, Dimitrios Gourgoulis, Constantine A. Stassinopoulos and Charalambos Yakinthos
"Impulse breakdown of positive direct voltage pre-stressing rod conductor gaps under negative impulse voltages"
35th Universities Power Engineering Conference, pp. 272, Belfast, 2000 Jan
[Abstract][BibTex]

@conference{Giagridis2000,
author={Z D. Giagridis and Dimitrios Gourgoulis and Constantine A. Stassinopoulos and Charalambos Yakinthos},
title={Impulse breakdown of positive direct voltage pre-stressing rod conductor gaps under negative impulse voltages},
booktitle={35th Universities Power Engineering Conference},
pages={272},
address={Belfast},
year={2000},
month={01},
date={2000-01-01}
}

Dimitrios Gourgoulis, Constantine A. Stassinopoulos and Charalambos Yakinthos
"The breakdown of negative pre-stressed conductor-plane gaps under positive switching impulse voltages"
35th Universities Power Engineering Conference, pp. 271, Belfast, 2000 Jan
[Abstract][BibTex]

@conference{Gourgoulis2000,
author={Dimitrios Gourgoulis and Constantine A. Stassinopoulos and Charalambos Yakinthos},
title={The breakdown of negative pre-stressed conductor-plane gaps under positive switching impulse voltages},
booktitle={35th Universities Power Engineering Conference},
pages={271},
address={Belfast},
year={2000},
month={01},
date={2000-01-01}
}

1999

Journal Articles

N. L. Allen and Pantelis N. Mikropoulos
IEEE Transactions on Dielectrics and Electrical Insulation, 6, (3), pp. 357-362, 1999 Jun
[Abstract][BibTex][pdf]

Results concerning the propagation of streamers along insulating surfaces under uniform electric field are presented. The basic properties of streamers, namely the electric field required for a stable propagation and the propagation velocity, have been measured and compared with propagation in air alone as a reference. The results have shown that in the experimental arrangement used significant space charges due to streamer branching are absent, therefore the properties observed are considered as characteristic of a single streamer. Streamers propagate stably with an intrinsic propagation field and a characteristic velocity that depend on the nature of the insulating material. For electric fields higher than the minimum field required for a stable propagation, a streamer system propagates with a \'surface\' and an \'air\' component

@article{allen1999,
author={N. L. Allen and Pantelis N. Mikropoulos},
title={Streamer propagation along insulating surfaces},
journal={IEEE Transactions on Dielectrics and Electrical Insulation},
volume={6},
number={3},
pages={357-362},
year={1999},
month={06},
date={1999-06-01},
url={http://ieeexplore.ieee.org/document/775623/?arnumber=775623},
doi={http://10.1109/94.775623},
abstract={Results concerning the propagation of streamers along insulating surfaces under uniform electric field are presented. The basic properties of streamers, namely the electric field required for a stable propagation and the propagation velocity, have been measured and compared with propagation in air alone as a reference. The results have shown that in the experimental arrangement used significant space charges due to streamer branching are absent, therefore the properties observed are considered as characteristic of a single streamer. Streamers propagate stably with an intrinsic propagation field and a characteristic velocity that depend on the nature of the insulating material. For electric fields higher than the minimum field required for a stable propagation, a streamer system propagates with a \\'surface\\' and an \\'air\\' component}
}

N L Allen and Pantelis N. Mikropoulos
Journal of Physics D: Applied Physics, 32, (8), pp. 913, 1999 Jan
[Abstract][BibTex][pdf]

Results concerning the dynamics of streamer propagation in air under a uniform electric field are presented and discussed. Experiments were performed in a plane-parallel electrode arrangement with positive streamers initiated at a sharp point in the earthed anode. The basic properties of streamers are described in terms of the electric field required for a stable propagation and the associated propagation velocity. Critical parameters are the ambient electric field, the voltage used for streamer initiation and the distance of traverse. The present experiments permit the separation between the effects of the above parameters upon streamer advancement and propagation over the whole path up to the cathode. It is shown that an intrinsic propagation field with an associated velocity can be defined, which determine the propagation of streamers of a limiting, minimum energy. The propagation velocity is a power function of the electric field and, with the aid of an empirical equation, values can be expressed accurately in terms of these intrinsic streamer properties.

@article{Allen1999PDAP,
author={N L Allen and Pantelis N. Mikropoulos},
title={Dynamics of streamer propagation in air},
journal={Journal of Physics D: Applied Physics},
volume={32},
number={8},
pages={913},
year={1999},
month={01},
date={1999-01-01},
url={http://iopscience.iop.org/article/10.1088/0022-3727/32/8/012/meta},
abstract={Results concerning the dynamics of streamer propagation in air under a uniform electric field are presented and discussed. Experiments were performed in a plane-parallel electrode arrangement with positive streamers initiated at a sharp point in the earthed anode. The basic properties of streamers are described in terms of the electric field required for a stable propagation and the associated propagation velocity. Critical parameters are the ambient electric field, the voltage used for streamer initiation and the distance of traverse. The present experiments permit the separation between the effects of the above parameters upon streamer advancement and propagation over the whole path up to the cathode. It is shown that an intrinsic propagation field with an associated velocity can be defined, which determine the propagation of streamers of a limiting, minimum energy. The propagation velocity is a power function of the electric field and, with the aid of an empirical equation, values can be expressed accurately in terms of these intrinsic streamer properties.}
}

1999

Conference Papers

N L. Allen and Pantelis N. Mikropoulos
"Streamer properties in air in the presence of insulators"
Electrostatics 99, Institute of Physics, pp. 49-52, Cambridge, UK, 1999 Jan
[Abstract][BibTex]

@conference{Allen1999EIP,
author={N L. Allen and Pantelis N. Mikropoulos},
title={Streamer properties in air in the presence of insulators},
booktitle={Electrostatics 99, Institute of Physics},
pages={49-52},
address={Cambridge, UK},
year={1999},
month={01},
date={1999-01-01}
}

N L. Allen and Pantelis N. Mikropoulos
"Profile effect on surface flashover in a uniform field"
11th International Symposium on High Voltage Engineering, London, UK, 1999 Jan
[Abstract][BibTex]

@conference{Allen1999ISHVE,
author={N L. Allen and Pantelis N. Mikropoulos},
title={Profile effect on surface flashover in a uniform field},
booktitle={11th International Symposium on High Voltage Engineering},
address={London, UK},
year={1999},
month={01},
date={1999-01-01}
}

N L. Allen and Pantelis N. Mikropoulos
"Influence of insulator profile on streamer propagation"
11th International Symposium on High Voltage Engineering, London, UK, 1999 Jan
[Abstract][BibTex]

@conference{Allen1999Mikropoulos,
author={N L. Allen and Pantelis N. Mikropoulos},
title={Influence of insulator profile on streamer propagation},
booktitle={11th International Symposium on High Voltage Engineering},
address={London, UK},
year={1999},
month={01},
date={1999-01-01}
}

1998

Journal Articles

N.L. Allen, D.E. Gourgoulis, Pantelis N. Mikropoulos, Constantine A. Stassinopoulos and C.G. Yakinthos
IEE Proceedings - Science, Measurement and Technology, 145, pp. 105-109, 1998 May
[Abstract][BibTex][pdf]

The sparkover of a 50 cm conductor–rod gap under positive impulses is investigated when a pre-stressing negative direct voltage is applied to the rod. Under lightning impulses the pre-stress has a small effect on the breakdown voltage of the gap. For long-front impulses, a considerable increase in the dielectric strength of the gap can be observed. Several parameters affect this behaviour, like the value of the pre-stressing direct voltage, the diameter of the energised conductor, the position of the rod with regard to the conductor and the waveshape of the applied impulse.

@article{allen1998,
author={N.L. Allen and D.E. Gourgoulis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos and C.G. Yakinthos},
title={Effects of negative direct voltage pre-stressing on the breakdown of conductor–rod gaps under positive impulse voltages},
journal={IEE Proceedings - Science, Measurement and Technology},
volume={145},
pages={105-109},
year={1998},
month={05},
date={1998-05-01},
url={http://digital-library.theiet.org/content/journals/10.1049/ip-smt_19981964},
issn={1350-2344},
abstract={The sparkover of a 50 cm conductor–rod gap under positive impulses is investigated when a pre-stressing negative direct voltage is applied to the rod. Under lightning impulses the pre-stress has a small effect on the breakdown voltage of the gap. For long-front impulses, a considerable increase in the dielectric strength of the gap can be observed. Several parameters affect this behaviour, like the value of the pre-stressing direct voltage, the diameter of the energised conductor, the position of the rod with regard to the conductor and the waveshape of the applied impulse.}
}

D.E. Gourgoulis and Constantine A. Stassinopoulos
IEE Proceedings - Science, Measurement and Technology, 145, pp. 147-151(4), 1998 Jul
[Abstract][BibTex][pdf]

The influence of irradiation on the breakdown of sphere and sphere–rod gaps for short gap spacings, has been investigated under standard impulse waveshapes of both polarities. It is known that, for short gap spacings, in non-irradiated sphere gaps and sphere–rod gaps the values of the standard deviations of the breakdown probability distributions are relatively large, and the same is true for the influence of humidity on the sparkover voltage. However, the experiments showed that irradiation acts interactively with other discharge parameters, i.e. impulse shape, gap spacing, polarity, absolute humidity, diameter of the spheres and the shape of the earthed electrode. It was found that the breakdown mechanism of small gaps is influenced by irradiation and that the irradiated gaps in most cases become more reliable for measuring and/or calibrating purposes.

@article{gourgoulis1998,
author={D.E. Gourgoulis and Constantine A. Stassinopoulos},
title={Influence of irradiation on impulse breakdown of sphere gaps and sphere–rod gaps},
journal={IEE Proceedings - Science, Measurement and Technology},
volume={145},
pages={147-151(4)},
year={1998},
month={07},
date={1998-07-01},
url={http://digital-library.theiet.org/content/journals/10.1049/ip-smt_19982105},
issn={1350-2344},
keywords={influence of humidity;sphere diameter;impulse shape;sphere gaps;sphere-rod gaps;sparkover voltage;standard impulse waveshapes;short gap spacings;breakdown probability distributions;electron avalanche;polarity;earthed electrode shape;breakdown mechanism;impulse breakdown;irradiation influence},
abstract={The influence of irradiation on the breakdown of sphere and sphere–rod gaps for short gap spacings, has been investigated under standard impulse waveshapes of both polarities. It is known that, for short gap spacings, in non-irradiated sphere gaps and sphere–rod gaps the values of the standard deviations of the breakdown probability distributions are relatively large, and the same is true for the influence of humidity on the sparkover voltage. However, the experiments showed that irradiation acts interactively with other discharge parameters, i.e. impulse shape, gap spacing, polarity, absolute humidity, diameter of the spheres and the shape of the earthed electrode. It was found that the breakdown mechanism of small gaps is influenced by irradiation and that the irradiated gaps in most cases become more reliable for measuring and/or calibrating purposes.}
}

Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
IEE Proceedings - Science, Measurement and Technology, 145, pp. 141-146, 1998 Jul
[Abstract][BibTex][pdf]

An attempt is made to assess the extent of influence of various parameters on the breakdown mechanism in short rod–plane gaps under positive impulses. Such parameters include front duration and peak value of the impulse voltage, gap length, profile of the rod and, in particular, absolute humidity. Depending on the above parameters, breakdown occurs either through a streamer-dominated breakdown mechanism or through one involving the development of a small leader. The second corona was found to predesignate which mechanism will take place, thus influencing the statistical behaviour of breakdown. Calculated humidity correction factors were found to be different from those proposed by IEC.

@article{mikropoulos1009PSMT,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Impulse breakdown of short rod–plane gaps and the influence of humidity},
journal={IEE Proceedings - Science, Measurement and Technology},
volume={145},
pages={141-146},
year={1998},
month={07},
date={1998-07-01},
url={http://digital-library.theiet.org/content/journals/10.1049/ip-smt_19982106},
abstract={An attempt is made to assess the extent of influence of various parameters on the breakdown mechanism in short rod–plane gaps under positive impulses. Such parameters include front duration and peak value of the impulse voltage, gap length, profile of the rod and, in particular, absolute humidity. Depending on the above parameters, breakdown occurs either through a streamer-dominated breakdown mechanism or through one involving the development of a small leader. The second corona was found to predesignate which mechanism will take place, thus influencing the statistical behaviour of breakdown. Calculated humidity correction factors were found to be different from those proposed by IEC.}
}

1998

Conference Papers

N L. Allen and Pantelis N. Mikropoulos
"On streamer propagation along insulating surfaces"
CIGRE 33-98 (WG04/07), IWD, 1998 Jan
[Abstract][BibTex]

@conference{Allen1998CIGRE,
author={N L. Allen and Pantelis N. Mikropoulos},
title={On streamer propagation along insulating surfaces},
booktitle={CIGRE 33-98 (WG04/07), IWD},
year={1998},
month={01},
date={1998-01-01}
}

Dimitrios Gourgoulis and Constantine A. Stassinopoulos
"Uncertainties of HV measurements using the Sphere and Rod gaps"
CIGRE 33-98 (WG03/07), IWD, 1998 Jan
[Abstract][BibTex]

@conference{Gourgoulis1998CIGRE,
author={Dimitrios Gourgoulis and Constantine A. Stassinopoulos},
title={Uncertainties of HV measurements using the Sphere and Rod gaps},
booktitle={CIGRE 33-98 (WG03/07), IWD},
year={1998},
month={01},
date={1998-01-01}
}

Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
"Humidity effect on the properties of coronas preceeding breakdown in short rod-plane gaps"
33rd Universities Power Engineering Conference, pp. 169, Edinburgh, 1998 Jan
[Abstract][BibTex]

@conference{Mikropoulos1998UPEC,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Humidity effect on the properties of coronas preceeding breakdown in short rod-plane gaps},
booktitle={33rd Universities Power Engineering Conference},
pages={169},
address={Edinburgh},
year={1998},
month={01},
date={1998-01-01}
}

Constantine A. Stassinopoulos
"Points causing uncertainty in sphere gap breakdown"
CIGRE 33-98 (WG03/41), IWD, 1998 Jan
[Abstract][BibTex]

@conference{Stassinopoulos1998,
author={Constantine A. Stassinopoulos},
title={Points causing uncertainty in sphere gap breakdown},
booktitle={CIGRE 33-98 (WG03/41), IWD},
year={1998},
month={01},
date={1998-01-01}
}

1997

Journal Articles

D. E. Gourgoulis, Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
IEE Proceedings - Science, Measurement and Technology, 144, (1), pp. 11-16, 1997 Jan
[Abstract][BibTex][pdf]

The relative low cost of sphere-rod gaps compared to sphere gaps makes it worthwhile to study the breakdown mechanisms of the former, if only to find whether they could be used for calibrating or measuring purposes, Having this in mind, the influence of impulse shape, gap spacing and humidity on the sparkover voltage of sphere-rod gaps always in comparison with sphere gaps has been investigated under standard impulse waveshapes. It is found that breakdown is influenced by humidity, waveshape and especially by polarity. With these factors in view, the breakdown mechanism and the feasibility of using them as measuring substandards is discussed

@article{gourgoulis,
author={D. E. Gourgoulis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Analysis of sphere-rod gaps under standard lightning and switching impulse voltages},
journal={IEE Proceedings - Science, Measurement and Technology},
volume={144},
number={1},
pages={11-16},
year={1997},
month={01},
date={1997-01-01},
url={http://ieeexplore.ieee.org/document/575878/?arnumber=575878},
keywords={electric breakdown;impulse shape;sphere-rod gaps;sparkover voltage;polarity;air gaps;breakdown probability curves;lightning;measurement standards;switching;transients;voltage measurement;2.5 to 15 cm;breakdown mechanisms;gap spacing;humidity influence;measuring substandards;standard lightning voltages;switching impulse voltages},
abstract={The relative low cost of sphere-rod gaps compared to sphere gaps makes it worthwhile to study the breakdown mechanisms of the former, if only to find whether they could be used for calibrating or measuring purposes, Having this in mind, the influence of impulse shape, gap spacing and humidity on the sparkover voltage of sphere-rod gaps always in comparison with sphere gaps has been investigated under standard impulse waveshapes. It is found that breakdown is influenced by humidity, waveshape and especially by polarity. With these factors in view, the breakdown mechanism and the feasibility of using them as measuring substandards is discussed}
}

D.E. Gourgoulis and Constantine A. Stassinopoulos
"Influence of humidity on the pre-breakdown phenomena in rod-plane gaps under impulse voltages with long wavetails"
Facta Universitatis, Series: Electronics and Energetics, 10, (2), pp. 281-294, 1997 Jan
[Abstract][BibTex]

@article{Gourgoulis1997,
author={D.E. Gourgoulis and Constantine A. Stassinopoulos},
title={Influence of humidity on the pre-breakdown phenomena in rod-plane gaps under impulse voltages with long wavetails},
journal={Facta Universitatis, Series: Electronics and Energetics},
volume={10},
number={2},
pages={281-294},
year={1997},
month={01},
date={1997-01-01}
}

D.E. Gourgoulis, Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
"On the corona inception in medium length positive rod-plane gaps under impulse voltages with long wavetails"
Facta Universitatis, Series: Electronics and Energetics, 10, (1), pp. 91-105, 1997 Jan
[Abstract][BibTex]

@article{gourgoulis1997FUEE,
author={D.E. Gourgoulis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={On the corona inception in medium length positive rod-plane gaps under impulse voltages with long wavetails},
journal={Facta Universitatis, Series: Electronics and Energetics},
volume={10},
number={1},
pages={91-105},
year={1997},
month={01},
date={1997-01-01}
}

D.E. Gourgoulis and Constantine A. Stassinopoulos
IEE Proceedings - Science, Measurement and Technology, 144, pp. 273-279(6), 1997 Nov
[Abstract][BibTex][pdf]

Sparkover voltages were measured for quasiuniform field sphere gaps consisting of 75 cm diameter spheres stressed by standard waveshapes of 1.2/50 µs and 250/2500 µs of both polarities. The work was undertaken with the aims of comparison with existing work with 25 cm spheres, and of investigating the breakdown mechanism of such gaps and the effect upon them of the applied impulse voltage, with emphasis on the waveshape, the gap spacing, the polarity, the diameter of the spheres and the absolute humidity. The factors that influence the breakdown mechanism have been studied systematically, in order to ascertain their reliability as measurement standards and, hence, their effect on the sparkover voltage values proposed by the IEC.

@article{gourgoulis1997SMT,
author={D.E. Gourgoulis and Constantine A. Stassinopoulos},
title={Spark breakdown of sphere gaps stressed by standard impulse voltages},
journal={IEE Proceedings - Science, Measurement and Technology},
volume={144},
pages={273-279(6)},
year={1997},
month={11},
date={1997-11-01},
url={http://digital-library.theiet.org/content/journals/10.1049/ip-smt_19971471},
issn={1350-2344},
abstract={Sparkover voltages were measured for quasiuniform field sphere gaps consisting of 75 cm diameter spheres stressed by standard waveshapes of 1.2/50 µs and 250/2500 µs of both polarities. The work was undertaken with the aims of comparison with existing work with 25 cm spheres, and of investigating the breakdown mechanism of such gaps and the effect upon them of the applied impulse voltage, with emphasis on the waveshape, the gap spacing, the polarity, the diameter of the spheres and the absolute humidity. The factors that influence the breakdown mechanism have been studied systematically, in order to ascertain their reliability as measurement standards and, hence, their effect on the sparkover voltage values proposed by the IEC.}
}

D. E. Gourgoulis, Pantelis N. Mikropoulos, Constantine A. Stassinopoulos and C. G. Yakinthos
IEE Proceedings - Science, Measurement and Technology, 144, (5), pp. 209-214, 1997 Sep
[Abstract][BibTex][pdf]

Results were obtained and evaluated so as to investigate the influence of various parameters on the breakdown mechanism of a 50 cm positive conductor-rod gap under impulse voltages. For such a gap it was found that several parameters affect its breakdown characteristics, notably humidity, waveshape of the applied impulse, diameter of the energised conductor and position of the earthed rod with respect to the conductor. Breakdown probability curves have been established, gap factors computed and their dependence on the aforementioned parameters investigated. Finally, the paths taken by the spark channel have been studied. Based on the information gained several suggestions concerning the breakdown mechanism are proposed

@article{gourgoulis97,
author={D. E. Gourgoulis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos and C. G. Yakinthos},
title={Behaviour of positive conductor-rod gaps stressed by impulse voltages in atmospheric air},
journal={IEE Proceedings - Science, Measurement and Technology},
volume={144},
number={5},
pages={209-214},
year={1997},
month={09},
date={1997-09-01},
url={http://ieeexplore.ieee.org/document/620457/?arnumber=620457},
keywords={sparkover voltage;breakdown mechanism;air gaps;overvoltage;power system transients;50 cm;atmospheric air;breakdown probability curves;earthed rod position;energised conductor diameter;gap factors;gap geometry;humidity effect;impulse voltage stressed;impulse waveshape effect;positive conductor-rod gaps;spark channel;streamer lengths},
abstract={Results were obtained and evaluated so as to investigate the influence of various parameters on the breakdown mechanism of a 50 cm positive conductor-rod gap under impulse voltages. For such a gap it was found that several parameters affect its breakdown characteristics, notably humidity, waveshape of the applied impulse, diameter of the energised conductor and position of the earthed rod with respect to the conductor. Breakdown probability curves have been established, gap factors computed and their dependence on the aforementioned parameters investigated. Finally, the paths taken by the spark channel have been studied. Based on the information gained several suggestions concerning the breakdown mechanism are proposed}
}

1997

Conference Papers

Dimitrios Gourgoulis and Constantine A. Stassinopoulos
"Effects of irradiation on the sparkover of sphere gaps under standard lightning impulses"
10th International Symposium on High Voltage Engineering, pp. 505-508, Montreal, Canada, 1997 Jan
[Abstract][BibTex]

@conference{Gourgoulis1997ISHVE,
author={Dimitrios Gourgoulis and Constantine A. Stassinopoulos},
title={Effects of irradiation on the sparkover of sphere gaps under standard lightning impulses},
booktitle={10th International Symposium on High Voltage Engineering},
pages={505-508},
address={Montreal, Canada},
year={1997},
month={01},
date={1997-01-01}
}

1996

Journal Articles

D. E. Gourgoulis, Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
IEE Proceedings - Science, Measurement and Technology, 143, (3), pp. 187-194, 1996 May
[Abstract][BibTex][pdf]

The influence of impulse shape, gap spacing and of humidity on the sparkover voltage of sphere gaps consisting of 25 cm diameter spheres has been studied for both polarities under standard waveshapes of 1.2/50 ?s and 250/2500 ?s. Results are compared with the values proposed by the IEC standard and an attempt to explain the breakdown mechanism and especially how it is influenced both by humidity and the waveshape has been made. Further suggestions are given concerning their use as calibrating gaps, especially under switching impulses. Also, under certain conditions, the inclusion in the calibrating gaps of 15 cm gaps is proposed

@article{gourgoulis96,
author={D. E. Gourgoulis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Sparkover voltage of sphere gaps under standard lightning and switching impulse voltages},
journal={IEE Proceedings - Science, Measurement and Technology},
volume={143},
number={3},
pages={187-194},
year={1996},
month={05},
date={1996-05-01},
url={http://ieeexplore.ieee.org/document/502438/?arnumber=502438},
abstract={The influence of impulse shape, gap spacing and of humidity on the sparkover voltage of sphere gaps consisting of 25 cm diameter spheres has been studied for both polarities under standard waveshapes of 1.2/50 ?s and 250/2500 ?s. Results are compared with the values proposed by the IEC standard and an attempt to explain the breakdown mechanism and especially how it is influenced both by humidity and the waveshape has been made. Further suggestions are given concerning their use as calibrating gaps, especially under switching impulses. Also, under certain conditions, the inclusion in the calibrating gaps of 15 cm gaps is proposed}
}

1996

Conference Papers

Dimitrios Gourgoulis, Constantine A. Stassinopoulos and Charalambos Yakinthos
"Impulse breakdown of positive conductor-rod gaps"
31st Universities Power Engineering Conference, pp. 1-4, Herakleion, Greece, 1996 Jan
[Abstract][BibTex]

@conference{Gourgoulis1996Stassinopoulos,
author={Dimitrios Gourgoulis and Constantine A. Stassinopoulos and Charalambos Yakinthos},
title={Impulse breakdown of positive conductor-rod gaps},
booktitle={31st Universities Power Engineering Conference},
pages={1-4},
address={Herakleion, Greece},
year={1996},
month={01},
date={1996-01-01}
}

Dimitrios Gourgoulis, Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
"On the breakdown parameters of medium rod-plane gaps stressed by positive impulse voltages with long wavetails"
31st Universities Power Engineering Conference, pp. 1-4, Herakleion, Greece, 1996 Jan
[Abstract][BibTex]

@conference{Gourgoulis1996UPEC,
author={Dimitrios Gourgoulis and Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={On the breakdown parameters of medium rod-plane gaps stressed by positive impulse voltages with long wavetails},
booktitle={31st Universities Power Engineering Conference},
pages={1-4},
address={Herakleion, Greece},
year={1996},
month={01},
date={1996-01-01}
}

1995

Conference Papers

N L. Allen, M Boutlentj and Constantine A. Stassinopoulos
"Gap factor and the breakdown mechanism in the conductor-rod geometry"
9th International Symposium on High Voltage Engineering, pp. 2096-1 – 2096-4, Graz, 1995 Jan
[Abstract][BibTex]

@conference{Allen1995Boutlentj,
author={N L. Allen and M Boutlentj and Constantine A. Stassinopoulos},
title={Gap factor and the breakdown mechanism in the conductor-rod geometry},
booktitle={9th International Symposium on High Voltage Engineering},
pages={2096-1 – 2096-4},
address={Graz},
year={1995},
month={01},
date={1995-01-01}
}

N L. Allen, M Boutlentj, Constantine A. Stassinopoulos and Charalambos Yakinthos
"A new phenomenon occurring in the breakdown of pre-stressed conductor-rod gaps under impulse voltages"
9th International Symposium on High Voltage Engineering, pp. 2936-1 – 2936-4, Graz, 1995 Jan
[Abstract][BibTex]

@conference{Allen1995ISHVE,
author={N L. Allen and M Boutlentj and Constantine A. Stassinopoulos and Charalambos Yakinthos},
title={A new phenomenon occurring in the breakdown of pre-stressed conductor-rod gaps under impulse voltages},
booktitle={9th International Symposium on High Voltage Engineering},
pages={2936-1 – 2936-4},
address={Graz},
year={1995},
month={01},
date={1995-01-01}
}

D E. Gourgoulis and Constantine A. Stassinopoulos
"On the influence of humidity on the impulse sparkover of sphere gaps"
Workshop on contemporary problems in power engineering, pp. 1-4, Thessaloniki, Greece, 1995 Jan
[Abstract][BibTex]

@conference{Gourgoulis1995WCPPE,
author={D E. Gourgoulis and Constantine A. Stassinopoulos},
title={On the influence of humidity on the impulse sparkover of sphere gaps},
booktitle={Workshop on contemporary problems in power engineering},
pages={1-4},
address={Thessaloniki, Greece},
year={1995},
month={01},
date={1995-01-01}
}

Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
"Humidity influences on the breakdown of od-plane gaps under positive impulses in atmospheric air"
Workshop on contemporary problems in power engineering, pp. 1-5, Thessaloniki, Greece, 1995 Jan
[Abstract][BibTex]

@conference{Mikropoulos1995WCPPE,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Humidity influences on the breakdown of od-plane gaps under positive impulses in atmospheric air},
booktitle={Workshop on contemporary problems in power engineering},
pages={1-5},
address={Thessaloniki, Greece},
year={1995},
month={01},
date={1995-01-01}
}

1994

Journal Articles

Pantelis N. Mikropoulos and Constantine A. Stassinopoulos
IEE Proceedings - Science, Measurement and Technology, 141, (5), pp. 407-417, 1994 Sep
[Abstract][BibTex][pdf]

To better understand the influence of atmospheric humidity on the breakdown mechanism of medium length rod-plane gaps when stressed by positive impulse voltages, emphasis was given to the initial prebreakdown phenomena (the first and second coronas) as well as the dark period. The salient events have been measured and assessed, especially the variation of their statistical distribution. Results were also taken and assessed to determine the influence of humidity on the breakdown probability curves, on the 50% breakdown voltages and on the distribution of the times to breakdown. During this investigation the main parameters, in addition to humidity, were the gap length, the geometry of the end of the rod and the front duration of the impulse voltage. Humidity was found to have a great influence on the breakdown mechanism, especially its streamer region. It influences not only the 50% breakdown voltage but also its standard deviation. It was also found to cause changes in the shape of the U-curves. The necessary correction for humidity was found to vary considerably with the front duration of the applied impulse

@article{Mikropoulos1994,
author={Pantelis N. Mikropoulos and Constantine A. Stassinopoulos},
title={Influence of humidity on the breakdown mechanism of medium length rod-plane gaps stressed by positive impulse voltages},
journal={IEE Proceedings - Science, Measurement and Technology},
volume={141},
number={5},
pages={407-417},
year={1994},
month={09},
date={1994-09-01},
url={http://ieeexplore.ieee.org/document/338819/?arnumber=338819},
abstract={To better understand the influence of atmospheric humidity on the breakdown mechanism of medium length rod-plane gaps when stressed by positive impulse voltages, emphasis was given to the initial prebreakdown phenomena (the first and second coronas) as well as the dark period. The salient events have been measured and assessed, especially the variation of their statistical distribution. Results were also taken and assessed to determine the influence of humidity on the breakdown probability curves, on the 50% breakdown voltages and on the distribution of the times to breakdown. During this investigation the main parameters, in addition to humidity, were the gap length, the geometry of the end of the rod and the front duration of the impulse voltage. Humidity was found to have a great influence on the breakdown mechanism, especially its streamer region. It influences not only the 50% breakdown voltage but also its standard deviation. It was also found to cause changes in the shape of the U-curves. The necessary correction for humidity was found to vary considerably with the front duration of the applied impulse}
}

1994

Conference Papers

N L. Allen, M Boutlentj and Constantine A. Stassinopoulos
"Parameters affecting the gap factor in the conductor – rod geometry"
CIGRE 33-94, WG 07, 22 IWD, pp. 1-8, Mulhouse, 1994 Jan
[Abstract][BibTex]

@conference{Allen1994Boutlentj,
author={N L. Allen and M Boutlentj and Constantine A. Stassinopoulos},
title={Parameters affecting the gap factor in the conductor – rod geometry},
booktitle={CIGRE 33-94, WG 07, 22 IWD},
pages={1-8},
address={Mulhouse},
year={1994},
month={01},
date={1994-01-01}
}

1992

Conference Papers

Constantine A. Stassinopoulos, Pantelis N. Mikropoulos and C J. Bagavos
"Positive impulse correction factor for medium rod-plane gaps"
X International Conference on Gas Discharges and their Applications, pp. 600-603, Swansea, 1992 Jan
[Abstract][BibTex]

@conference{Stassinopoulos1992,
author={Constantine A. Stassinopoulos and Pantelis N. Mikropoulos and C J. Bagavos},
title={Positive impulse correction factor for medium rod-plane gaps},
booktitle={X International Conference on Gas Discharges and their Applications},
pages={600-603},
address={Swansea},
year={1992},
month={01},
date={1992-01-01}
}

Constantine A. Stassinopoulos, Pantelis N. Mikropoulos and C J. Bagavos
"On the influence of humidity on positive impulse breakdown of medium rod-plane gaps"
X International Conference on Gas Discharges and their Applications, pp. 596-599, Swansea, 1992 Jan
[Abstract][BibTex]

@conference{Stassinopoulos1992ICGDA,
author={Constantine A. Stassinopoulos and Pantelis N. Mikropoulos and C J. Bagavos},
title={On the influence of humidity on positive impulse breakdown of medium rod-plane gaps},
booktitle={X International Conference on Gas Discharges and their Applications},
pages={596-599},
address={Swansea},
year={1992},
month={01},
date={1992-01-01}
}

1990

Journal Articles

Constantine A. Stassinopoulos, K. Andreadou and A Sergaki
"Influence of humidity on small rod-plane gap breakdown"
ETEP, etz-Archiv, 12, pp. 273-277, 1990 Jan
[Abstract][BibTex]

@article{stassinopoulos1990,
author={Constantine A. Stassinopoulos and K. Andreadou and A Sergaki},
title={Influence of humidity on small rod-plane gap breakdown},
journal={ETEP, etz-Archiv},
volume={12},
pages={273-277},
year={1990},
month={01},
date={1990-01-01}
}

1988

Conference Papers

Constantine A. Stassinopoulos
"Impulse breakdown of sphere gaps"
IX International Conference on Gas Discharges and their Applications, pp. 463-466, Venice, 1988 Jan
[Abstract][BibTex]

@conference{Stassinopoulos1998ICGDA,
author={Constantine A. Stassinopoulos},
title={Impulse breakdown of sphere gaps},
booktitle={IX International Conference on Gas Discharges and their Applications},
pages={463-466},
address={Venice},
year={1988},
month={01},
date={1988-01-01}
}