2023
Journal Articles
Ioannidis Alexios I., Datsios Zacharias G., Gerodimos Apostolos K. and Tsovilis Thomas E.
IEEE Transactions on Electromagnetic Compatibility, pp. 1-11, 2023 Jan
[Abstract][BibTex][pdf] Lightning attachment can be modeled through a stochastic approach adopting a detailed representation of the lightning phenomenon. A fractal-based modeling technique can be used for this purpose, considering lightning discharge branched and tortuous behavior, as well as physical properties associated with downward and upward leaders’ inception and propagation. However, fractal-based simulations require substantial computational resources, especially for the accurate calculation of the electric field at all points of the discretized simulation domain at each simulation step. Thus, the considerable computational cost inhibits the extensive application of stochastic simulations for estimating lightning incidence to common structures and power systems. This work investigates optimization techniques for fractal-based simulations regarding total simulation time; these are applicable to both high-performance computing and personal computers. The proposed techniques consist of a C-MATLAB integration methodology, as well as a multi-color ordering algorithm enabling parallel execution using CPU and GPU programming. Applications associated with lightning incidence to overhead transmission lines are presented. The total simulation time is substantially reduced with respect to the original code. A reduction of up to 98% is achieved, enhancing the applicability of stochastic modeling to lightning incidence estimation problems. @article{IOANNIDIS2023TEMC, | |
Staikos Evangelos T., Peppas Georgios D. and Tsovilis Thomas E.
IEEE Transactions on Power Delivery, 38, (1), pp. 453-462, 2023 Feb
[Abstract][BibTex][pdf] The voltage-current characteristic of metal-oxide varistors is experimentally investigated for a wide frequency range. Concave exponential and rectangular impulse currents that resemble lightning current field records are employed to investigate their energy coordination with transient voltage suppression diodes, commonly integrated into printed circuit boards to protect sensitive electronic components. Thus, the effective coordination is experimentally evaluated against high- and low-frequency surge events for components used in power, telecom, and automotive applications. Experimental results supported by ATP-EMTP simulations show that reliable coordination of conventional varistors with bidirectional diodes can be challenging; this finding reveals a risk for the uninterrupted operation and safety of equipment with built-in diodes. The satisfactory agreement of ATP-EMTP results with experimental data shows that electromagnetic transient simulations can be an effective tool for designing coordination circuits and evaluating the risk of failure in case of surge events and electromagnetic pulses. @article{STAIKOS2023TPWRD, |
2022
Journal Articles
Zacharias G. Datsios, Pantelis N. Mikropoulos, Thomas E. Tsovilis, Emmanuel Thalassinakis and Grigorios Pagonis
Electric Power Systems Research, 213, pp. 108763, 2022 Jan
[Abstract][BibTex][pdf] This paper investigates the application of Line Surge Arresters (LSAs) to 150 kV double-circuit overhead lines through ATP-EMTP simulations. The 150 kV lines under study are the backbone of the autonomous electrical system of Rhodes island in southeastern Aegean Sea, Greece. Hence, lightning-related insulation flashovers causing outages of these lines may lead to a blackout of this isolated system. In this work, both Non-Gapped Line Arresters (NGLAs) and Externally Gapped Line Arresters (EGLAs) are evaluated. Two installation configurations with 3 and 6 LSAs per tower are considered protecting one circuit and both circuits of the line, respectively. Simulations were performed for both lightning strikes to phase conductors and tower; these may cause shielding failure flashover and backflashover, respectively. The conducted current through the LSAs and the voltage at their terminals are computed, enabling the estimation of the energy stressing the LSAs. The effects of the lightning current time to half value, the phase angle of the operating voltage, and the power frequency ground resistance of towers are assessed. The required LSA energy absorption capability is determined. The lightning performance of a critical overhead line of the Rhodes 150 kV system is estimated and its improvement due to LSA installation is quantified. @article{DATSIOS2022EPSR, | |
Martins-Britto Amauri G., Papadopoulos Theofilos A., Datsios Zacharias G., Chrysochos Andreas I. and Papagiannis Grigoris K.
IEEE Transactions on Electromagnetic Compatibility, 64, (6), pp. 2273-2282, 2022 Dec
[Abstract][BibTex][pdf] The electromagnetic interference (EMI) of overhead transmission lines (OHLs) to aboveground pipelines sharing the same right-of-ways has been a research topic for many years. Most studies focus on investigations of the induced currents and voltages at the mains frequency (50/60 Hz). However, significant voltage levels can also be induced, when the pipeline is subject to high-frequency excitation. The representation of earth conduction effects is very important to estimate accurately the level of the EMI. This article presents a detailed high-frequency analysis of OHL/pipeline configurations taking into consideration the frequency-dependent (FD) soil properties. Wise's earth formulation is adopted for the calculation of the earth return impedances and shunt admittances. Propagation characteristics, frequency-domain, and transient responses are calculated by using both the FD soil models and constant soil properties, as well as the classical transmission line approach of Carson. Frequency-domain responses calculated with Wise's formulation are compared against those obtained using the MoM-SO method of EMTP software. @article{DATSIOS2022TEMC, | |
Datsios Zacharias G., Mikropoulos Pantelis N., Staikos Evangelos T., Tsovilis Thomas E., Patsalis Diamantis G., Vlachopoulos Dimitrios and Ganatsios Stergios
IEEE Transactions on Industry Applications, 58, (1), pp. 212-223, 2022 Jan
[Abstract][BibTex][pdf] Soil ionization, that is, electrical discharges developing in the ground, may have a significant impact on the behavior of grounding systems under lightning currents, as it causes a reduction of the impulse ground impedance. This may have several implications on insulation coordination studies for power systems. The effects of soil ionization have not been completely clarified yet due to its complexity associated with soil characteristics and conditions, ground electrode configuration, lightning current characteristics, as well as difficulties in the accurate determination of discharge activity. This study deals with the experimental investigation of the lightning impulse behavior of wet sandy soil. An experimental setup and measurement procedure are introduced for this purpose. Experiments under lightning impulse voltages of negative polarity were performed on two wet sandy soils, both as obtained from their extraction sites, as well as after drying and manual addition of moisture; thus, the influence of the soil sample preparation procedure is evaluated. Results are discussed considering soil ionization inception, electrical breakdown of soil, impulse impedance, and soil electrical properties. A simulation model is proposed for the prediction of the instantaneous impulse impedance when soil ionization occurs; this model is suitable for time domain simulations in ElectroMagnetic Transients Program (EMTP)-type software. @article{DATSIOS2022TIA, | |
Ioannidis Alexios I., Mikropoulos Pantelis N., Tsovilis Thomas E. and Kokkinos Nikolaos D.
IEEE Industry Applications Magazine, 28, (4), pp. 20-28, 2022 Jul
[Abstract][BibTex][pdf] A fractal-based approach to the lightning protection of historical buildings and monuments against direct lightning flashes is introduced. The proposed methodology is based on a stochastic lightning attachment model that considers physical criteria on downward and upward leaders’ inception and propagation. The model foresees multiple competing upward leaders incepting from vulnerable points of the monument under protection and considers the branching and tortuous behavior of the lightning attachment phenomenon. Applying the proposed methodology to the Parthenon has shown that the risk of direct lightning flashes per year that cause physical damage to the monument is not acceptable without an external lightning protection system (LPS); two different LPSs are evaluated in terms of lightning interception efficiency. Lightning risk assessment results are discussed and compared with those obtained by implementing the procedures of the International Electrotechnical Commission (IEC) 62305. @article{IOANNIDIS2022IAM, | |
D. Khelil, S.Bouazabia, and P. N. Mikropoulos
COMPEL - The international journal for computation and mathematics in electrical and electronic engineering, 41, (4), pp. 1159-1170, 2022 Feb
[Abstract][BibTex][pdf] The paper aims to estimate the instantaneous breakdown voltage of the lightning discharge from simulated figures in an energized rod-plane configuration protected by a lightning rod. The same configuration of electrodes has been the subject of experimental investigations for the measurement of the instantaneous breakdown voltage using oscillographic monitoring. This study validates the simulation model by making a comparison with experimentation and involves the role of the inception field of the upward discharge in the propagation of this last one. @article{Khelil2022COMPEL, | |
Emara Manal M., Peppas Georgios D., Pyrgioti Eleftheria C., Chronopoulos Demetrios D., Bakandritsos Aristides, Tegopoulos Sokratis N., Kyritsis Apostolos, Tsovilis Thomas E., Polykrati Aikaterini D. and Gonos Ioannis F.
IEEE Transactions on Dielectrics and Electrical Insulation, 29, (2), pp. 510-518, 2022 Apr
[Abstract][BibTex][pdf] This work reports on the significant enhancement of the thermal properties of the FR3 natural ester dielectric oil after the addition of pentyl-graphene nanosheets, as confirmed by thermal diffusivity and specific heat studies at different concentrations and temperatures (30 °C–90 °C). Experimental results of the dielectric constant demonstrated a constant value in the kHz region while increased in the low-frequency region, decreased with increased temperature, and saturated at 0.008% w/w concentration. In addition, light absorption is used for a better understanding of the properties variation upon changing graphene’s concentration as a method to estimate the agglomeration level. The optimum concentration for its best performance in terms of thermal and dielectric properties is 0.008% w/w, whereby the thermal diffusivity and the dielectric constant increased by 43.04% and 6.18%, respectively. @article{TSOVILIS2022TDEIS, |
2021
Journal Articles
T.A. Papadopoulos, Z.G. Datsios, A.I. Chrysochos, P.N. Mikropoulos and G.K. Papagiannis
Electric Power Systems Research, 196, pp. 107249, 2021 Jan
[Abstract][BibTex][pdf] In surge analysis, an important issue is the influence of the imperfect earth on the propagation characteristics of the conductors. In this paper, the wave propagation characteristics and the transient performance of underground multiconductor cable systems in flat, vertical and trefoil arrangement are investigated. The Longmire and Smith frequency-dependent (FD) soil model as well as a generalized earth formulation are considered, taking into account in the analysis the impact of earth conduction effects on both the series impedance and shunt admittance of the cable conductors and sheaths. Comparisons are carried out with approximate earth formulations, neglecting the influence of imperfect earth on shunt admittances. Finally, resonance frequency analysis and transient simulations are performed for the different cable arrangements to evaluate the importance of FD soil modeling and earth formulation per cable arrangement. @article{DATSIOS2021EPSR, | |
Datsios Zacharias G., Mikropoulos Pantelis N. and Tsovilis Thomas E.
IEEE Transactions on Power Delivery, 36, (2), pp. 522-532, 2021 Apr
[Abstract][BibTex][pdf] The minimum backflashover current, IBF, of typical overhead transmission lines is estimated through ATP-EMTP simulations considering the tower ground resistance and AC voltage phase angle as parameters. Several single- and double-circuit lines are investigated with operating voltage from 66 kV to 765 kV and Basic Insulation Level from 325 kV to 1950 kV. The backflashover rate, BFR, of the investigated lines is obtained using an expression that accurately considers lightning incidence to the line for prospective currents higher than IBF; the relevant CIGRE expression underestimates BFR. BFR can be easily estimated using solely the phase angle values corresponding to zero AC voltage of the phase conductors. Based on the IBF and BFR results, simple expressions are introduced for the calculation of an equivalent minimum backflashover current as a function of insulator length and tower ground resistance or impulse impedance. These expressions enable straightforward estimation of BFR without the need for time-consuming simulations or laborious analytical methods adopting simplifying assumptions. @article{DATSIOS2021TPWRD, | |
Datsios Zacharias G., Ioannidis Alexios I., Papadopoulos Theofilos A. and Tsovilis Thomas E.
IEEE Transactions on Electromagnetic Compatibility, 63, (5), pp. 1433-1443, 2021 Oct
[Abstract][BibTex][pdf] A stochastic lightning attachment model for evaluating the lightning performance of overhead transmission lines is introduced. The proposed model considers the stochastic nature of lightning attachment by taking into account the lightning discharge branched and tortuous behavior. It integrates physical criteria on leader inception by adopting the Petrov and Waters model, as well as leader discharge progression considering also the competing interaction of multiple upward leaders. A generalized methodology combining stochastic modeling of lightning incidence with an electromagnetic transients (EMT) simulation model is proposed so as to estimate the backflashover and shielding failure flashover rates of HVDC overhead lines; an application to a monopolar −400 kV line sheds light on the basic parameters that affect the lightning performance of HVDC lines. The probability of shielding failure is higher than that obtained by the deterministic lightning attachment models. The stochastic nature of lightning attachment and the effect of lightning peak current distribution should be considered for the design and evaluation of the lightning performance of the emerging HVDC systems. @article{IOANNIDIS2021TEMC, | |
Ioannidis Alexios I. and Tsovilis Thomas E.
IEEE Transactions on Industry Applications, 57, (3), pp. 2317-2325, 2021 May
[Abstract][BibTex][pdf] A fractal-based approach is presented for the direct-stroke shielding analysis of high-voltage substations. The proposed method considers the effect of lightning polarity and the stochastic nature of the lightning attachment phenomenon. An application to a typical 69 kV substation reveals that shielding failures may occur even if the shielding design is realized with IEEE Std 998-2012 methods. This can be explained by the fact that the shielding design methods adopted by IEEE Std 998 do not consider the effects of lightning polarity and object height on striking distance. In addition, IEEE Std 998 methods adopt a deterministic approach on shielding failure analysis neglecting the branched and tortuous behavior of lightning discharges. Fractal-based simulation results show a considerable dispersion of striking distance and a significantly higher shielding failure probability for downward positive lightning than negative lightning. This work forms a discussion framework on shielding failure analysis following a stochastic approach; this approach may explain unexpected shielding failures, such as those reported in IEEE surveys, and can be used for the direct-stroke shielding design of mission-critical high-voltage substations. @article{IOANNIDIS2021TIA, | |
Papadopoulos Theofilos A., Datsios Zacharias G., Chrysochos Andreas I., Mikropoulos Pantelis N. and Papagiannis Grigoris K.
IEEE Transactions on Electromagnetic Compatibility, 63, (1), pp. 259-267, 2021 Feb
[Abstract][BibTex][pdf] The accurate estimation of the influence of the imperfect earth on the propagation characteristics of conductors is a crucial issue in electromagnetic (EM) transient analysis. In this aspect, the frequency-dependence of the soil electrical properties should be also considered. The effects of the dispersion of soil electrical properties on EM propagation have been investigated mostly for overhead transmission lines. This article presents a detailed EM transient analysis of underground cable systems taking into consideration the frequency-dependent (FD) soil properties. A generalized earth formulation of cable earth return impedance and shunt admittance is adopted. Propagation characteristics and transient responses are calculated by using both FD soil models and constant soil properties, as well as approximate earth formulations, neglecting the influence of the imperfect earth on shunt admittances; significant differences are observed. Guidelines for the accurate evaluation of earth conduction effects on the transient performance of underground cable systems are also introduced. @article{MIKROPOULOS2021TEMC, | |
Tsovilis Thomas E.
IEEE Transactions on Power Delivery, 36, (5), pp. 3055-3064, 2021 Oct
[Abstract][BibTex][pdf] Surge protective devices, SPDs, improve the reliability of power systems by mitigating the risk of equipment failure due to impinging overvoltages. The role of SPDs is more challenging today when considering the advent of smart grids and the associated integration of sensitive electronic equipment into modern low-voltage power systems. This work provides critical insight into performance requirements and test methods for surge protective devices according to IEC 61643 and UL 1449 standards; SPD ratings and fail-safe behavior are on focus. Experimental arrangements are proposed to represent in a laboratory environment the surge current and/or temporary overvoltage events that SPDs may be exposed to in the field; experimental evidence, ATP-EMTP simulation results and field experience are used to support these proposals. Grey zones of SPDs performance and failure mode are discussed in the context of the integrated components technology. @article{TSOVILIS2021TPWRD, |
2020
Journal Articles
Evanthia I. Bousiou, Pantelis N. Mikropoulos and Vasileios N. Zagkanas
Electric Power Systems Research, 178, pp. 106032, 2020 Jan
[Abstract][BibTex][pdf] Corona inception field of line conductors is of major importance for assessing corona performance of overhead transmission lines. Using computational models that consider the ionization processes in corona discharge, the corona inception field of ACSR conductors typically utilized in overhead transmission lines is estimated. The interactive effects of line conductor geometry, voltage polarity as well as atmospheric conditions on corona inception field are assessed and discussed based on the ionization area around the conductor. It is shown that the corona inception field of bundle configurations can be satisfactorily approximated by that of the subconductor when considered single. Generally, the inception field of negative corona is slightly lower than that of positive corona and lesser affected by variations in atmospheric conditions. An empirical expression for the straightforward estimation of the corona inception field of line conductors has been introduced, accounting well for the effects of conductor geometry, voltage polarity and atmospheric conditions. @article{BOUSIOU2020EPSR, | |
Topcagic Zumret, Mlakar Miha and Tsovilis Thomas E.
IEEE Transactions on Power Delivery, 35, (3), pp. 1180-1188, 2020 Jun
[Abstract][BibTex][pdf] The energy absorption capability of metal-oxide varistors, decidedly influenced by the electrothermal processes occurring within their microstructures, is a vital parameter for the effective surge protection. This paper proposes a numerical model for the simulation of the electrothermal phenomena occurring in metal-oxide varistors. The model is based on a 3D representation of the varistor's polycrystalline microstructure and the finite element method analysis. With the aid of the proposed model, the interdependence of the varistor's energy absorption capability, nonuniform current conduction, and the thermal behavior is investigated. Results are in good alignment with experimental data and discussed in the context of electrothermal properties of the varistor's microstructure. An unambiguous dependency of the varistor's energy handling capability on the applied overvoltage is shown to exist. A quantitative evaluation of the effective volume decrease due to the severe current localization at temporary overvoltages is demonstrated. The presented work analyses varistor's current conduction under temporary overvoltages and provides the means for extension of the varistors IV characteristics into the time domain so as to accurately predict their dynamic electrothermal behavior. The introduced dynamic (IV t) model may have significant implications in energy absorption requirements and design principles for surge protective devices. @article{TSOVILIS2020TPWRD, | |
Tsovilis Thomas E. and Topcagic Zumret
IEEE Transactions on Power Delivery, 35, (5), pp. 2541-2543, 2020 Oct
[Abstract][BibTex][pdf] This letter provides evidence of a peculiar behavior of metal-oxide varistors under DC overvoltages. It stresses the need for extending the V-I characteristic of varistors in the time domain so as to accurately predict the dynamic performance of varistor-based surge protective devices under temporary overvoltages. Observations and mathematical formulation of DC overload behavior of metal-oxide varistors may have significant implications in energy absorption requirements for low-voltage surge protective devices and unlocks possible monitoring strategies in emerging DC distribution smart grids. @article{ZUMRET2020TPWRD, |
2019
Journal Articles
Datsios Z. G. and Mikropoulos P. N.
IEEE Transactions on Dielectrics and Electrical Insulation, 26, (3), pp. 904-912, 2019 Jun
[Abstract][BibTex][pdf] The electrical properties of soil at low radio-frequencies are important for engineering applications. In this work the effects of excitation frequency (42 Hz-1 MHz) as well as soil characteristics and conditions on the electrical properties of soil are experimentally investigated using remolded soil samples. Several sandy soils were tested with variable water content (dry up to saturation) using guarded two-electrode arrangements. A method is proposed based on the Nyquist plot to estimate the critical frequency above which measurements of the electrical properties of wet soil are free from electrode polarization effects. The latter result in extremely high real permittivity values, which do not correspond to the response of soil. The critical frequency is higher for smaller grain size and higher water content and varies according to a power law with soil power-frequency conductivity. The real and (effective) imaginary relative permittivity decrease with increasing frequency whereas the effective conductivity increases; this is attributed to the distributed relaxation of the interfacial polarization mechanism associated with the particulate nature of soil. The values of soil electrical properties increase with increasing water content and decreasing grain size (increasing specific surface) as a consequence of enhanced polarization and conduction phenomena. The effective imaginary permittivity varies with frequency according to a power law with an exponent approximately −1; the multiplier coefficient of this law expresses losses and has been formulated as a function of the degree of saturation and soil specific surface. @article{DATSIOS2019TDEIS, | |
Datsios Zacharias G., Mikropoulos Pantelis N. and Tsovilis Thomas E.
IEEE Transactions on Electromagnetic Compatibility, 61, (3), pp. 623-630, 2019 Jun
[Abstract][BibTex][pdf] Modeling of lightning return-stroke channel affects significantly the estimation of fast-front overvoltages arising across overhead line insulators, thus, also the lightning performance of power lines. This paper introduces a simplified method for estimating the minimum lightning currents causing shielding failure flashover and backflashover of overhead transmission line insulation, as affected by lightning channel equivalent impedance. The validity of the proposed methodology is demonstrated through ATP-EMTP simulations of typical 66 up to 765-kV overhead transmission lines with variable tower ground resistance. An assessment is made of the effects of lightning channel equivalent impedance on the lightning performance of overhead lines, that is, shielding failure flashover and backflashover rates. Lightning channel equivalent impedance values of 1000 and 400 Ω are suggested, respectively, for shielding failure and backflashover studies, based on the results obtained in this paper together with a discussion on the literature data regarding channel impedance. @article{DATSIOS2019TEMC, | |
I. F. Gonos, P. N. Mikropoulos, and P. Svarnas
High Voltage, 4, (3), pp. 159-160, 2019 Sep
[Abstract][BibTex][pdf] @article{Mikropoulos2018ICHVESel, | |
Mikropoulos Pantelis N. and Tsovilis Thomas E.
IEEE Transactions on Electromagnetic Compatibility, 61, (6), pp. 1793-1802, 2019 Dec
[Abstract][BibTex][pdf] General closed-form expressions for estimating lightning incidence to telecommunication towers are introduced on the basis of several lightning attachment models. The expressions consider the effects of tower height and lightning peak current distribution, as well as orographic and shielding effects. The estimated annual number of lightning strikes to towers, varying considerably among lightning attachment models, increases with tower height according to a power law and it is higher for lightning peak current distributions with high median value and standard deviation. A comparison is made between estimates and field data on lightning incidence; IEC geometric method, which is traditionally used in industry, by exaggerating the effect of tower height, overestimates lightning incidence. Literature lightning attachment models suitable for estimating lightning incidence to telecommunication towers are suggested based also on the observed failure rate of a transformer feeding a GSM base station. @article{TSOVILIS2019TEMC, |
2018
Journal Articles
Djazia Khelil, Slimane Bouazabia and Pantelis.N Mikropoulos
Journal of Electrostatics, 95, pp. 42-52, 2018 Jan
[Abstract][BibTex][pdf] An experimental investigation on the lightning discharge interception probability of a vertical grounded rod when inserted in a rod-plane gap, as affected by the presence of a neighboring grounded object (rod) is presented. The height of the two grounded rods (simulating a protective and a protected object against lightning) and their separation distance and placement with respect to the energized rod were considered as influencing parameters on breakdown voltage and time, thus also on the discharge interception probability, under negative lightning impulse voltages. The discharge interception probability of the vertical grounded rod is strongly affected by the height and position of the neighboring rod; it significantly reduces when the separation distance between rods decreases, especially as the height of the rods increases. In the presence of the neighboring rod breakdown to the vertical rod occurs sooner at higher instantaneous voltages. A fractal model describing the lightning interception probability in the evaluated electrode arrangement is developed; the electrical discharges, emanating from both energized and grounded electrodes, evolve randomly in the space where the electric field is most intense. By considering also the voltage drop along the discharge path at each evolution step a satisfactory agreement between simulation and experimental results has been achieved. @article{KHELIL2018JELECTROSTAT, | |
Zumret Topcagic, Thomas Tsovilis and Dejan Krizaj
Electric Power Systems Research, 164, pp. 253-262, 2018 Jan
[Abstract][BibTex][pdf] Nonuniform current distribution inside varistor ceramics is a key factor influencing its performance and failures. Therefore understanding, modeling and predicting of current distribution in varistor ceramics is of crucial significance. This paper proposes a numerical model for simulation of nonuniform electric current distribution inside zinc oxide varistors. A numerical model is based on physical modeling of the varistor's grain-structured geometry presented by Voronoi network using finite element method (FEM) simulation. The presented method is solving complete electric field inside the modeled geometry and therefore provides a more physically accurate approach for better understanding and predicting nonuniform current distribution in the varistor. In order to properly establish a FEM model a novel approach in defining grain boundary characteristic is proposed. Thus, a macroscopic model of the varistor microstructure has been developed and the grain micro-junction boundary characteristic has been derived. The simulation results of nonuniform current distribution in a varistor agree well with measurement results for a typical ZnO varistor. The presented model enables investigation of influences of varistor geometry (shapes, sizes) and material properties on the current distribution. A new mathematical expression for varistor I–V characteristic based on Lambert function is proposed. @article{TOPCAGIC2018EPSR, |
2017
Journal Articles
Zacharias G. Datsios and Pantelis N. Mikropoulos
Electric Power Systems Research, 150, pp. 36-44, 2017 Jan
[Abstract][BibTex][pdf] Typical grounding configurations are used in MV/LV distribution substations often without evaluating their safety performance against the danger of critical electric shock due to touch and step voltages arising in case of a ground fault. A method for the straightforward safety assessment of typical grounding configurations of MV/LV substations is introduced on the basis of simple calculations. A safety performance curve is constructed by using proportionality factors, specific to each typical grounding configuration, and the time–current characteristic of the installed protective device. The safety performance curve relates ground fault current to upper limits of soil resistivity, thus also ground resistance, below which safety is ensured. Thus, safety of an existing or new MV/LV substation can be easily evaluated through the associated safety performance curve. In a similar approach, a method is introduced to determine the shortest separation distance between MV/LV substation and LV neutral grounding configurations ensuring safety against transferred potentials where common grounding is not applied. The use of the proposed methods is demonstrated through an application to typical 20/0.4kV distribution substations. @article{DATSIOS2017EPSR, | |
Datsios Z. G., Mikropoulos P. N. and Karakousis I.
IEEE Transactions on Dielectrics and Electrical Insulation, 24, (5), pp. 3063-3072, 2017 Oct
[Abstract][BibTex][pdf] Soil resistivity is an important parameter for engineering applications. In this work the effects of soil characteristics and conditions on the DC soil resistivity are investigated experimentally using remolded soil samples. Several natural silica and artificial glass sands of uniform gradations were tested under variable water resistivity and content, using the two-electrode and the four-electrode soil box methods for dry and wet soil, respectively. The dependence of DC soil resistivity on soil characteristics and conditions, such as grain size, specific surface, pore water conductivity and degree of saturation, is assessed. Soil resistivity decreases with increasing water conductivity and content. It also decreases with decreasing grain size as a result of the shorter current conduction paths (lower tortuosity) and the higher amount of available electrolytes; the contribution of the latter was quantified by measuring the resistivity of several soil:water suspensions. The effects of soil characteristics and conditions on soil resistivity have been well accounted for by considering soil resistivity at saturation as reference. For degree of saturation higher than the percolation threshold soil resistivity is well approximated by Archie's law; parameter values applicable to sandy soil are proposed. A new expression has been derived on a physical basis to describe soil resistivity for degree of saturation values lower than the percolation threshold. @article{DATSIOS2017TDEIS, | |
Mavrikakis N.C., Mikropoulos P.N. and Siderakis K.
IEEE Transactions on Dielectrics and Electrical Insulation, 24, (1), pp. 490-498, 2017 Feb
[Abstract][BibTex][pdf] Condition assessment of naturally aged composite insulators, aiming to better understand the ageing processes involved, is essential for maintaining high reliability in power networks. In this study field-ageing effects on insulating materials (EPDM and VMQ/HCR) of two 150 kV composite suspension insulators are evaluated. The insulators had been in service for 17 years on a coastal transmission network, exposed to seaborne salt spray, windborne soil deposits and intense UV solar radiation. Several diagnostic techniques, including visual inspection, wettability classification, FTIR spectroscopy, SEM and EDX analysis were employed to assess the condition of the insulators. Field-ageing effects were not assessed as critical to dictate insulator replacement. For both insulators morphological and material degradation due to field-ageing was restricted to the upper surface layer of the polymeric housing. The resistance of housing material against tracking and erosion remained unaffected by field-ageing. Surface deterioration of EPDM was more intense than that of VMQ/HCR, despite the fact that the EPDM insulator had been in operation at a site with a lower pollution severity. The EPDM housing was classified as hydrophilic; however, after removing pollutant deposits its hydrophobicity was improved. The hydrophobic properties of the VMQ/HCR housing were only slightly reduced. Field-ageing effects on insulator housing are discussed based on material structural changes caused by ageing stresses. @article{MAVRIKAKIS2017TDEIS, |
2016
Journal Articles
Zacharias G. Datsios and Pantelis N. Mikropoulos
Electric Power Systems Research, 139, pp. 37-46, 2016 Jan
[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{DATSIOS2016EPSR, | |
Mikropoulos P. N. and Zagkanas V. N.
IEEE Transactions on Dielectrics and Electrical Insulation, 23, (3), pp. 1322-1330, 2016 Jun
[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{ZAGKANAS2016TDEI, |
2015
Journal Articles
Pantelis N. Mikropoulos and Vasileios N. Zagkanas
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, |
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, | |
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, |
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, |
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, | |
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, |
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, |
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, | |
Lazaros Lazaridis and Pantelis N. Mikropoulos
IET Sci. Meas. Technol, 4, (2), pp. 71-83, 2010 Mar
[Abstract][BibTex][pdf] 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, | |
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, | |
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, | |
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][pdf] 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, | |
Pantelis N. Mikropoulos and Thomas Tsovilis
IEEE Trans. on Power Delivery, 25, (3), pp. 1855-1865, 2010 Jun
[Abstract][BibTex][pdf] 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, |
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, |
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, | |
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, | |
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, | |
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, |
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, | |
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, |
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, |
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, | |
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, |
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, | |
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, | |
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, |
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, | |
D.E. Gourgoulis and Constantine A. Stassinopoulos
Facta Universitatis, Series: Electronics and Energetics, 10, (2), pp. 281-294, 1997 Jan
[Abstract][BibTex][pdf] @article{Gourgoulis1997, | |
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, | |
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, | |
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, |
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, |
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, |
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, |