Standard Lightning Impulse Voltage Research Articles

Voltage and Current Characteristics of Schottky Barrier Diodes and Rectifier Diodes Exposed to Standard Lightning Impulse Voltage

Lightning surges are one of the major reasons of bypass diode (BPD) failure in photovoltaic modules. In this study, standard lightning impulse voltages (SLIVs) were directly applied to Schottky barrier diodes (SKB diodes) and rectifier diodes (PNJ diodes) with different electrical specifications. The critical voltages Vc of SLIV at which the diodes were damaged and the temporal variations of diode current (iD) and voltage (vD) were measured. It was found the iD‐vD characteristics can be categorized into a total of eight patterns: four patterns for the forward SLIV application and the four for the reverse SLIV application, regardless of diode types. The measured results imply that the irreversible voltage dip that occurs before the termination of the reverse breakdown contributes significantly to diode damage in the both forward and reverse SLIV applications. SKB diodes exposed to the reverse SLIV are easily damaged at the lowest magnitude of SLIV among all combinations of diode types and SLIV directions. Once a diode has been damaged due to the first SLIV application, the second application of SLIV to the same diode causes remarkable change in the iD‐vD characteristic even if the magnitude of the second SLIV is smaller than Vc. © 2024 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Combined effect of lightning impulse voltage and temperature stress on the propagation of creeping discharge of oil-impregnated paper

This article presents the results of an experiment designed to study the impact of temperature on the characteristic parameters of creeping discharges. The insulating interfaces consist of a thermally enhanced cellulose surface immersed in mineral oil, palm kernel oil methyl ester (PKOME) and castor oil methyl ester (COME). The study was carried out under a standard negative lightning impulse voltage (1.2/50 μs). The article also presents the complete algorithms for calculating the maximum extension of the discharges, the ionisation rate and the charge produced by them. The results of the study show that temperature favors the propagation of discharges and the ionisation rate. It was observed that liquids with a higher dielectric constant and high electrical conductivity were more exposed to the impact of temperature. The results show ionisation increments of 0.973%/°C, 1.093%/°C and 1.076%/°C in mineral oil (MO), COME and PKOME respectively. The maximum extension of the discharges shows a linear evolution with the applied voltage and temperature but a non-linear increment with the temperature. As for the charge produced, it shows a constant increment with temperature and voltage in each liquid. These values are (5.839%/°C, 1.977%/kV), (6.047%/°C, 2.082%/kV) and (6.177%/°C, 2.113%/kV) respectively in MO, COME and PKOME.

Lightning Impulse Voltage Stresses In Underground Cables

Underground power cables are crucial for transmission and distribution. Lightning can stress their insulation, but not directly. So, impulse cable testing is studied. This research examines the cable’s transient response to standard and non-standard lightning impulse voltage waveforms. MATLAB Simulink was used to model a 132 kV wire with standard and non-standard impulse voltages. The IEC60060-1(2010) lightning impulse test uses a conventional waveform impulse voltage with a front time and a tail time of 1.2/50μs half value, while the non-standard test uses a front time and a tail time of 0.8/12s half value. Non-standard impulse waveforms are more accurate than standard waveforms. The impulse test voltage is four to five times the underground cable’s operational voltage and must withstand five applications without damage. Standard and non-standard impulse waveforms are injected with 132 kV and 550 kV to evaluate insulation failure or damage. Standard lightning and non-standard impulse voltage waveforms do not cause insulation failure or damage. When 132 kV and 550 kV are introduced into the normal and non-standard lightning impulse waveforms, the overshoot voltage increases. The peak voltage of a non-standard 550 kV impulse voltage waveform exceeds the IEC impulse withstand voltage. The finding shows that non-standard impulse voltage waves create increased cable voltage stress.

Comparison of negative streamer development in synthetic ester and mineral oil in a point-sphere gap divided by a pressboard barrier

This article presents the results of the studies on development of negative streamers in synthetic ester and mineral oil with electrode system using a pressboard barrier. The tests were performed at standard lightning impulse voltage in point-sphere electrode configuration as reference system and a 5 mm thick pressboard barrier placed in different locations between the point and the sphere. Introducing pressboard between the electrodes allowed to relate the observations to the real insulating systems where long oil gaps are divided on shorter by insulating barriers. It was done to show whether pressboard barrier changes the streamers behavior in the considered electrode gap distance what is important from the viewpoint of assessment of dielectric properties of alternative liquids versus mineral oils. The results showed that the pressboard barrier for both liquids tested prevents breakdown. In addition, the barrier causes reduction of the propagation velocity of the streamers in synthetic ester, which is benefit in relation to the results concerning free liquid gaps.

Breakdown Characteristics of Double Vacuum Gaps in Series Under Lightning Impulse Voltage: Optimal Gap Distance Distribution

An optimal distribution of gap distance for the double vacuum gaps (VGs) in series can improve its dielectric strength. The objective of this article is to quantitatively determine the optimal distribution of gap distance for the double VGs in series. The double VGs were built up by vertically connecting two pairs of copper sphere-plane electrodes in series. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{u{\%{}}}$ </tex-math></inline-formula> was the percentage of the upper VG distance in the total gap distance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{T}$ </tex-math></inline-formula> of 5 mm. A negative standard lightning impulse voltage (LIV) was applied in the dielectric test. The experimental results reveal that, under <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{u{\%{}}}$ </tex-math></inline-formula> of 70%, the breakdown (BD) voltage of double VGs reached the maximum 192.7 kV, which was 13.2% higher than that of a single VG with the same <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{T}$ </tex-math></inline-formula> . Therefore, the optimal gap distance distribution of double VGs was determined to be 70%/30% in this experiment. Moreover, when <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{u{\%{}}}$ </tex-math></inline-formula> was less than 40% or greater than 90%, the dielectric strength of double VGs was mainly determined by the VG with a longer distance. When <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{u{\%{}}}$ </tex-math></inline-formula> was in the range of 40%–90%, the dielectric strength of double VGs can be supported by both VGs and significantly improved. The dielectric strength should reach the maximum on the condition that the maximum electric field strengths at the cathode of two VGs were balanced.

Insulation Reinforcement for Cable Connector of CRH2 Electric Multiple Unit Under Standard Lightning Impulse Voltage

In order to improve the insulation capacity of cable connector (Cc) in the high voltage equipment box (Hveb), this article proposes an optimization scheme that does not change the size of the Hveb and the structure of Cc fittings. In this article, the change in the impulse voltage withstand value of Cc after installing insulating sheath was verified by the lightning impulse flashover test. Based on the finite-element software and the structure of Cc fittings, a special shaped shielding ring (Sr) was designed to adjust the partial area where the electric field intensity are concentrated. Besides, the optimal parameters of the Sr under different working conditions were determined by the orthogonal design. Finally, utilizing ultraviolet (UV) imaging technology and image processing method, the discharge intensity of Cc fittings were characterized. The results demonstrate that with the combination of insulation measures, the impulse voltage withstand value of Cc can be increased from 155 to 235 kV. The maximum electric field strength on the surface of bolts and metal electrode can be reduced by 57.8% and 72%; UV discharge facular area had an exponential relationship with the applied voltage, the initial discharge voltage of Cc can increase by 243% after installing the Sr.

がいしの塩汚損が配電線直撃雷によるスパークオーバ発生率に与える影響

To improve the lightning-outage-rate calculation method of medium-voltage distribution lines, it is important to accurately assess sparkover rates of insulators. In salt polluted areas, lightning outage rates may increase due to the deterioration of insulation performance of insulators. Although the sparkover characteristics of salt-polluted insulators against the standard lightning impulse voltage have been reported in previous works, the sparkover characteristics against short-wave-tail lightning impulse voltages, which occur across insulators on distribution lines by direct lightning strikes, still remain not clarified.

Electric Field Analysis on the Corona Discharge Phenomenon According to the Variable Air Space between the Ionizer and Ground Current Collector

In this paper, we present the optimized air space of the lightning protection rod (SK-AOR380) with the function of a charge transfer system (CTS). For evaluation of CTS in the laboratory setting, some studies have focused on the modification of the structure and shape of the CTS; the air space is designed (&gt;2 mm) as an empirical design without quantitative data. However, in this paper, we have focused on the air space between the ionizer conductor and current collector to control the inception and occurrence position of corona discharge in air insulation. This is because the performance, such as the initial corona discharge inception of CTS, is determined by the air space. The simulation analysis was performed in a narrow, micro-sized air space as a first step, where the air space was reduced to the extent possible for simulation. To evaluate the performance of SK-AOR380 according to the narrow air space, we considered the numerical analysis method. The fundamental equations consist of Poisson’s equation and the charge continuity equation. Poisson’s equation for electric fields is a fully coupled numerical model based on the charge continuity equations for a positively charged ion, negatively charged ion, and free electron. Fowler–Nordheim electron emission was employed for the boundary condition at the surface of the ionizer conductor. To simulate the corona discharge behavior under standard lightning impulse voltage, we used a source of lightning voltage with 1.2/50 μs based on a double exponential equation; the corona discharge behaviors (electric field distribution, free electron density, positive and negative ion density) were investigated dependent on each time step (0.5, 1 and 1.2 μs) until 3.5 μs. The results revealed that the characteristics graph of free electron density, positive and negative ion density showed similar trends, with lightning impulse voltage increasing with increasing time steps until 1.2 μs and each density resulted in a decreasing trend from 1.2 μs to 3.5 μs. The SK-AOR380 is improved with a decreasing air space in terms of electric field distribution, electron, and ion density. In other words, the 0.0005 mm air space created a non-uniform electric field distribution with a large field enhancement, causing ionization to initiate corona discharge. In addition, in the case of a 0.0005 mm air space, the electric field and electron density are increased by 1.3 and 1.9 times, respectively, than that of 0.001 mm. However, there was no longer a significant difference under 0.0005 mm in the simulation results. To improve the CTS, we suggest the air space between the ionizer conductor and current collector should be less than 2 mm than that of conventional CTS from our research work.

Partial Discharge Measurement and Analysis of Transformer Under Oscillating Lightning Impulse Voltage

Oscillating lightning impulse (OLI) voltage is recommended for on-site testing due to its high generation efficiency. In this article, partial discharge (PD) characteristics under field impulse voltage waveforms are investigated. The experiment is carried out on a scaled transformer model with a protrusion defect inside as the PD source. Both oscillating and standard lightning impulse (SLI) voltages are acquired as test waveforms. Main discharges and reverse discharges are detected simultaneously under both OLI and SLI voltage. The increasing applied voltage amplitude advances the main discharge and enlarges its amplitude, and it also prolongs the duration of the reverse discharge and increases the number. The amplitude of main discharge is related to the PD voltage, while that of reverse discharge depends on the dynamic variation of the impulse voltage. Since OLI voltage contains multiple rising edges during the oscillation period, more PDs are excited. A disruptive effect (DE) model is extended for the PD analysis to show the oscillation effect on the PD distribution. The OLI test with PD measurement performed on a 220-kV transformer proves that PD detection during impulse voltage withstand test reflects more detailed information that can be used for early diagnosis of power transformers.

Study of Creeping Discharges Propagating Along a Solid Insulating Surface Immersed in Mineral Oil and Palm Kernel Oil Methyl Ester Under Negative Lightning Impulse Voltage

This article presents the results of an experiment designed to study creeping discharges on a solid insulating surface immersed in palm kernel oil methyl ester (PKOME) compared with those produced from mineral oil (MO). The investigation takes place under standard negative lightning impulse voltage application, and for the three following solid insulating surfaces: hardened glass (HG), porcelain (PO), and simple glass (SG). The distributions of the radial discharge occupancy densities as a function of the thickness of the solid material and the insulating liquid are evaluated. A statistical study of the total and final length of the branches produced by the discharges is presented, as well as the current amplitudes and the charge produced by this phenomenon. A fractal analysis of the phenomenon is also presented. The experimental results show that the distribution of the average radial density of the discharge, the total and final lengths of the branches, and fractal dimension depend strongly on the physicochemical characteristics of the insulation system.

Breakdown characteristics of triple vacuum gaps in series under lightning impulse voltage: Effect of floating potential gap

Inside a high-voltage vacuum interrupter, multiple floating shields form the multiple vacuum gaps (VGs) in series. When the number of series VGs exceeds three, there will exist floating potential VGs in which both electrodes have floating potentials. The floating potential VG is important for the dielectric strength of multiple VGs in series. The objective of this paper is to quantitatively determine the effect of the floating potential VG distance on the breakdown (BD) characteristics of triple VGs in series. In the experiments, triple VGs were built up by series-connecting three pairs of sphere-plane electrodes vertically. A negative standard lightning impulse voltage (1.2/50 μs) was applied. The experimental results revealed that a critical gap distance of the floating potential VG was required to support the dielectric strength of triple VGs in series. The distance was approximately 0.7 mm in this experiment. The critical distance of the floating potential VG can be determined by an intersection of the maximum electric field strength at cathode and the 100% BD electric field strength of the floating potential VG. This method can be used to estimate the critical distance of the floating potential VG in a high-voltage vacuum interrupter.

Understanding the Lightning Impulse Discharge Characteristics of Nano Silica Modified Sunflower Oil for High-Voltage Insulation Applications

Vegetable oils with nanotechnology added to them are a possible replacement for mineral oils in power transformer applications today. Because of the size and production cost reductions as well as the oil's high level of biodegradability, this could result in an increase in the dielectric strength of insulating fluid. Since transformers are mainly used for outdoor applications, understanding the capability of insulating fluid to withstand lightning discharge is a vital issue. This study used experimental analysis to compare the lightning discharge performance of pure sunflower oil and sunflower oil modified with SiO2 (Nano) at various weight-per-volume concentrations of 0.01 %, 0.05 %, and 0.1 %. This experimental work was conducted with 1.2/50 µs of standard lightning impulse voltage with positive polarity. The result illustrated that the nano-modified sunflower oil had greater lightning impulse withstand strength than the pure sunflower oil.

Investigation of the corona discharge activity in liquid nitrogen under transient voltage conditions using fluorescent fiber sensor

In the present study, corona inception voltage in liquid nitrogen under standard lightning impulse voltage (SLIV) and oscillatory impulse voltages were studied using fluorescent fiber sensor. A single discharge signal is observed near the peak of the applied SLIV. The amplitude of the fluorescent signal is high with positive polarity than under negative polarity. At higher voltages, the magnitude of the fluorescence signal increases and it shifts towards the rising edge of the applied voltage. Under unidirectional oscillatory impulse voltage (UOIV) and sinusoidal bidirectional oscillatory impulse voltage (SBOIV) conditions, Corona discharge occurs around the peak of the first cycle. When the applied voltage is increased, the discharges occur in the subsequent cycles. Under switching impulse voltage (SIV), corona discharge is noticed at the rising edge in positive polarity, whereas in negative polarity, it is observed at the falling edge. The corona discharge inception voltage (CDIV) is determined to be highest under Unidirectional Oscillating Lightning Impulse (UOLI) and lowest under SI. The negative polarity exhibits more temporal spread than the positive polarity under oscillating impulse voltages.

Influence of Temperature on Lightning Performance of Mineral Oil

This article deals with the influence of temperature on the lightning impulse breakdown voltage (LI BDV) of mineral oil in a non-uniform electric field. The experiments were performed in point–plane electrode arrangements under a standard lightning impulse voltage of negative polarity. The LI BDV for the temperatures of 20, 40, 60, 80, 100 and 110 °C was measured using a step method with registration of the light signals emitted during pre-breakdown and breakdown phases. The average propagation velocity was also determined. The results confirm that the mineral oil tested under LI voltage behaves similarly to AC voltage-based stress. The LI BDV increases with temperature up to 100 °C and then tends to decrease. The results demonstrated that viscosity dependent processes are responsible for increasing the LI BDV with temperature up to the 100 °C; and liquid vapor formation processes are responsible for decreasing the LI BDV at temperatures equal to and higher than 100 °C.

Ice nucleation forced by transient electric fields.

Icing affects many technical systems, like aircraft or high-voltage power transmission and distribution in cold regions. Ice accretion is often initiated by ice nucleation in sessile supercooled water droplets and is influenced by several influencing factors, of which the impact of electric fields on ice nucleation is still not completely understood. Especially the influence of transient electric fields is rarely or not at all investigated, even though it is of great interest, e.g., for high-voltage transmission lines or for the food industry. In the present study the impact of transient electric fields on ice nucleation in supercooled sessile water droplets is experimentally investigated under well-defined conditions. A set of droplets is cooled down to a certain temperature and is subsequently exposed to electric fields generated from standard lightning or standard switching impulse voltages, which are commonly used for testing of high-voltage equipment. The nucleation behavior of individual droplets is captured using a high-speed camera and the effect of the transient electric field on ice nucleation is analyzed by considering both the singular and the stochastic nature of nucleation. While the singular nature of nucleation is referred to during analysis of the relative number of droplets remaining liquid long times after the impulse voltage, its stochastic nature is accounted for in the analysis of the temporal evolution of the relative number of frozen droplets. It is shown that low electric field strengths (E[over ̂]≤6.52kV/cm) only have a negligible impact on ice nucleation, independent of the supercooling. In contrast, high electric field strengths (E[over ̂]≥9.78kV/cm) promote significantly ice nucleation. It is also shown that depending on the supercooling, the freezing delay of the different droplets in the ensemble may vary over several magnitudes for the same conditions. It is demonstrated that the electric field appears to indirectly affect the nucleation rate by generating droplet oscillations, finally promoting ice nucleation. The experiments clearly demonstrate the possibility to actively force ice nucleation by applying transient electric fields. These results improve the understanding of ice accretion on high-voltage insulators and may also lend insight into freezing processes in food industry. We expect that these results will be a valuable contribution in formulating and/or validating new nucleation models.

Breakdown Voltage of Transformer Oil Containing Cellulose Particle Contamination With and Without Bridge Formation Under Lightning Impulse Stress

This study investigates the effect of cellulose bridge formation on insulating oil. Such effect influences the breakdown voltage and strength of mineral oil (MO) under standard lightning impulse (LI) voltage. A cylindrical test vessel fitted with spherical–spherical electrodes with a 2 mm gap distance between them was used to generate a quasi-uniform field and observe bridge formation between the gaps. A positive LI waveform (1.2/ $50~\mu \text{s}$ ) generated by an impulse generator was applied during the breakdown tests. The rising-voltage method was used to measure the breakdown voltage in accordance with the IEC 60897 standard test method. Weibull cumulative breakdown probability was applied to present the breakdown results statistically, and the outcome was compared with clean oil (CO). Commercial cellulose microcrystalline particles were dispersed into the transformer oil to act as bridge. The concentrations of the samples were 0.004%, 0.008% and 0.012% by weight. The breakdown test results showed that the cellulosic particles reduced the LI breakdown voltage (LIBV) of MO by up to 10%. The influence of cellulose particles on LIBV was also more prominent during bridge formation, with a further reduction of 14%. Evidently, the presence of the cellulose bridge exerted a detrimental effect on the breakdown voltage of MO under LI stress, yielding a reduction of 24% and 27% at 0.004 wt% and 0.012 wt%, respectively. Finite element analysis also indicated that the electrical field strength of cellulosic particle contamination of 0.004 wt% and 0.012 wt% without a bridge skeleton exceeded the critical electric field strength of CO by 20% and 61%, respectively. The presence of cellulosic contamination at any concentration, i.e. either with or without a bridge structure, clearly decreased the breakdown strength of transformer oil. However, the breakdown is not occurred instantaneously. The LI breakdown was initiated and established only at a certain impulse stress level, when the electric field strength along the bridge lines exceeded the critical electric field strength of the oil. Thus, this finding potentially contributes to the formulation of guidelines for condition assessment and contamination monitoring to minimise common issues in power transformer failures that are attributable to the insulation system, and consequently, achieve optimum transformer insulation integrity.

Dielectric properties of mixed mineral and synthetic ester oil

The dielectric properties of 20% synthetic ester oil mixed with mineral oil were studied for its potential applications as an insulator in transformers. The corona inception voltage of mixed oil under AC and DC voltages was measured by the ultra-high frequency (UHF) technique. A statistical study of the breakdown voltage of mixed oil under AC, DC and standard lightning impulse voltages was carried out to obtain a database which would be useful for designing transformer insulation. Further, the mixture was subjected to continuous breakdown studies under AC voltage with lightning impulse voltages, at regular intervals and characterised through measurement of interfacial tension, flash point, ionic mobility as well as UV–vis spectroscopy and frequency-domain spectroscopy studies to understand the liquid's chemical stability. The results of the study have shown that the mixture was stable even after several breakdowns. The ionic mobility and polarisation current have increased, indicating higher losses in the oil. UHF signal energy analysis and phase-resolved partial discharge analysis have revealed a reduction in discharge activity with mixed oils.

Investigation on thermally aged natural ester oil for real‐time monitoring and analysis of transformer insulation

Thermal aging of natural ester oil shows drastic reduction in partial discharge inception voltage (PDIV) and a significant variation is observed only above a certain aging time, under AC, DC, high frequency AC voltages and with harmonic voltages with different total harmonic distortion. Weibull distribution studies on PDIV measurements indicate a reduction in scale parameter (α) with increase in thermal aging temperature. A characteristic reduction in breakdown voltage was observed with the thermally aged ester oil, under AC, DC and standard lightning impulse voltage. The breakdown voltage variation with aged ester oil follows normal distribution. Ultraviolet (UV) analysis of ester oil thermally aged at 160°C has revealed a regular shift of the derived absorbance parameter to longer wavelengths. The interfacial tension and turbidity exhibits an inverse relationship with the thermally aged ester oil. Gas chromatography/mass spectrometric analysis of the thermally aged ester oil predicted the formation of more carboxylic acids and ketones with aging duration. The steady-state fluorescence on thermally aged ester oil exhibits a shift in its emission profile, which is in tandem with the UV absorption spectroscopic analysis. Fluorescence analysis can be adopted as a real-time monitoring tool in transformers, to understand the condition of liquid insulation. The viscosity dependence on the wavelength of derivative absorption maxima follows a direct relationship with the thermally aged natural ester oil.

Epoxy insulator with surface graded-permittivity by magnetron sputtering for gas-insulated line

To reduce the electric field concentration induced flashover in AC gas insulated line (GIL), a novel concept of surface functionally graded material (SFGM) is proposed in this paper, and the continuously graded high-A layer was fabricated by depositing BaTiO 3 on the insulator surface. The iterative method was introduced to optimize the thickness distribution of the BaTiO 3 layer. After four iterations, the layer thickness distribution almost reaches its optimal solution, and the maximum electric field strength is reduced by approximately 70% compared with that of the conventional insulator. To fabricate the optimized SFGM insulator, a rotatable baffle with the designed gap was placed above the insulator during the sputtering process. For comparison, a uniformly sputtered insulator with the 4μm BaTiO 3 layer and a discretely graded insulator with four gradients were prepared. Both electric field simulations and experiments were performed to evaluate the electrical performance of different insulators. The results show that, compared with the conventional insulator, the sputtered insulators, especially the optimized SFGM insulator, can significantly improve the uniformity of the electric field distribution along the insulator surface under a 50 Hz voltage. Accordingly, the discharge inception voltage and flashover voltage of the insulator both increased significantly. Under negative standard lightning impulse voltage (1.2/50 μs), the sputtered insulators show insulation performance comparable to that of the conventional insulator.

Breakdown characteristics of gas‐filled transformers and reactors under lightning and switching impulse voltages

This article discusses the breakdown voltage characteristics of gas‐filled transformers and reactors. To determine their insulation characteristics, this study has used models simulating their insulating elements: interturn and section‐to‐section insulation. The study measured 50% breakdown voltages in the section‐to‐section and interturn models under standard lightning impulse voltages that were 1.13 and 1.06 times higher than those under the standard switching impulse voltages, respectively. The mechanism of the above‐mentioned facts is discussed, in terms of the formative time lag of the discharge. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.