Lightning Impulse Withstand Voltage Research Articles

Aging Impact on Relative Permittivity, Thermal Properties, and Lightning Impulse Voltage Performance of Natural Ester Oil Filled With Semiconducting Nanoparticles

This study is focused on exploring the change in the dielectric and cooling efficiency of vegetable oil following the integration of semiconducting SiC and TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles in two different weight percentages by comparing the corresponding behavior of nanofluids at the time of their synthesis and after one month of ageing. The thermal response has been evaluated by means of diffusivity and conductivity, while dielectric properties are measured with non-destructive tests (relative permittivity and dissipation factor) and destructive ones in terms of lightning impulse withstand voltage. The results of the latter property measurements are also compared to numerical model findings. The analysis of results proves that the properties under study are worsened with increase in nanoparticles’ concentration, probably attributing it to quicker agglomeration, as it is also confirmed by the simulation results.

Research on the influence and suppression technology of micron foreign matter for gas insulated switchgear

Breaking–closing of the circuit breaker inevitably produces micron metal dust, significantly affecting the internal insulation state of the GIS. This paper first analyzes the force characteristics of micron metal dust and its separation characteristics inside the circuit breaker, then builds a test platform supporting 520 kV power frequency withstand voltage and 1175 kV lightning impulse withstand voltage based on true type 363 kV GIS to investigate the influence of micron foreign matter on the insulation performance of GIS. Subsequently, ANSOFT is used to simulate the electric field distortion when there is micron metal dust inside the circuit breaker, and the influence of foreign matter on the degree of electric field distortion under the action of the flow field. The characteristics of the influence of micron foreign matter on the insulation performance of foreign matter inside the GIS are summarized. Secondly, using artificial micron foreign matter, the pulse current and the UHF methods are adopted to detect the discharge amount. The sensitivity of different detection methods to the discharge detection of the micron foreign matter is analysed, plus the partial discharge characteristics of the micron foreign matter under different voltages. Afterwards, based on the test results of micron foreign matter, two improvement measures are proposed, including the treatment of foreign matter inside the GIS and the dual-light source angiography inspection, so as to reduce the influence of micron foreign matter on the insulation status of the GIS. Finally, through the AC withstand voltage test of artificial embedded foreign matter and the insulation margin check under 1.1 times lightning impact, the effectiveness of the measures proposed in this paper is verified, thus providing technical support for certifying the impact of micron foreign matter on the insulation state during equipment operation and maintenance.

Pushing the boundaries of inductive voltage transformer design

Designing inductive voltage transformers for the highest voltage levels (550 kV and above) is a true challenge indeed. The main reason for this is their complex internal R-L-C structure, including an insulating system which needs to be resistant to all types of overvoltages and consequent dielectric stress it can encounter during its lifetime. This is why it is necessary to verify behavior of such transformers during the design process, and under four sets of standard test overvoltage types; the Power Frequency Withstand Voltage (PFWV), the Lightning Impulse Withstand Voltage (LIWV, i.e. BIL), Chopped Impulse Withstand Voltage (CIWV) and the Switching Impulse Withstand Voltage (SIWV). The main idea of the paper is to demonstrate that by understanding the influence of crucial parameters of the appropriate equivalent diagram on the voltage distribution across the active part of the transformer, it is possible to define the overall design of inductive voltage transformers so that they can satisfy even the most rigorous insulation requirements, thus pushing the boundaries of design even further.

Impact of MV Standards on Shipboard DC Cable System Size

There is rising interest in medium-voltage direct current (MVDC) power systems for several reasons, including compatibility with DC loads and avoidance of alternating current (AC) frequency synchronization issues when combining multiple source outputs [1]. However, few MVDC systems are currently in operation, and there is relatively little experience compared to the knowledge available for medium-voltage alternating current (MVAC) systems. Presently, IEEE Std. 1709 is the only standard that provides guidance for MVDC systems [2]. This work&#x2019;s objective is to harmonize the information within MVAC standards to produce prospective design and test values for shipboard MVDC cables and evaluate their impact on cable system sizes. Standards for MVAC cable design and test parameters including Basic Lightning Impulse Insulation Level, Withstand Voltage, and cable insulation thickness are compiled to understand the range of parameter values deemed acceptable for MVAC cables. Based on existing shipboard MVAC standards and IEEE Std. 1709, this study produces prospective design and test values for a shipboard MVDC system with a 12 kV Nominal System Voltage. Additionally, a tradeoff was identified with the thickness of cable insulation where a small reduction in cable system size can be achieved but at the expense of substantially greater electric stresses within the insulation.

Conceptual Design and Feasibility Study of Flexible Gas-Insulated Transmission Line Using CF3I Gas Mixture

SF6 is widely used in high voltage equipment because of its outstanding characteristics as insulation medium. However, SF6 is a potent greenhouse gas, so its use and emission are regulated. CF3I has been investigated as a candidate for SF6 alternative gas, whose environmental impact is much less than that of SF6 and other fluorocarbon-based gases. In addition, a flexible gas-insulated transmission line (FGIL) is expected to be used for power transmission from offshore wind farms due to its high flexibility and low capacitance. In this paper, the feasibility of 22 kV class FGIL utilizing atmospheric pressure CF3I gas is evaluated from the viewpoint of withstand voltage with actual lightning impulse tests. The cross-sectional size of FGIL is determined by considering insulation condition, heat transfer condition, sheath electric field condition and spacer stress condition. The impulse tests for FGIL are conducted in straight and bent states, and it is revealed that the 50% breakdown voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> of straight state exceeded lightning impulse withstand voltage (LIWV) for 22 kV class, and that V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> of bent state falls below LIWV.

Analysis of VFTO during the failure of a 550-kV gas-insulated substation

This paper presents an analysis of a very fast transient overvoltage (VFTO) experienced by a 550-kV gas-insulated substation (GIS) that caused the failure of a disconnector switch (DS) and circuit breaker (CB), after manual closure of the DS. Simulations were performed using an electromagnetic transient program with GIS component modeling for high-frequency analysis and multi-spark electric-arc modeling. The simulation results were verified by comparison with field test measurements and showed an acceptably accurate waveform and amplitude prediction for VFTO on the 550-kV GIS under study, allowing for improved failure analysis. The simulation results using equivalent electrical circuit models indicated that the failures occurred at a voltage below the lightning impulse withstand voltage (LIWV) with a high quantity of sparks. After the analysis, it was possible to conclude that the manual closure of the DS generated excessive sparks, causing significant damage to the fixed and mobile contacts and abnormal stress in the insulation of SF6 gas in the inhomogeneous electric field region of the CB, impacting on the reduction of the dielectric strength of SF6 gas, thereby producing a high-intensity arc between the shield and the enclosure.

Insulation properties of C4F7N/CO2 mixtures under lightning impulse

C 4 F 7 N and mixtures have been intensively investigated around the world for the substitution of SF 6 in gas insulated transmission lines (GIL). In the present paper, the authors experimentally investigate the lightning impulse (LI) breakdown voltages of C 4 F 7 N/CO 2 in different electric field uniformities, temperatures and pressures. In a slightly nonuniform electric field, the insulation strength of 9%C 4 F 7 N/91%CO 2 can reach 80% strength of SF 6 at the same pressure. In a very un-uniform electric field, the pressure of 9%C 4 F 7 N/91%CO 2 must be increased to 0.7 MPa to keep 80% strength of the 0.4 MPa SF 6 . Also, the polarity effect occurred in different electric fields. The breakdown voltage of 9%C 4 F 7 N/91%CO 2 versus gas pressure shows an N-curve, which may be utilized for the design of GIL. At the pressure of 0.35∼0.7 MPa, 9%C 4 F 7 N/91%CO 2 gas mixtures in ultra-high voltage platform has passed the LI withstand voltage tests. Based on various temperature breakdown tests and the Antoine equation calculation, 9%C 4 F 7 N/91%CO 2 could work at −15 °C.

Study on Influence Factors of Lightning Impulse Test Waveform of UHV GIS Equipment

The lightning impulse withstand voltage test is a test item that must be carried out for type testing, factory test or even field test of electrical equipment such as gas insulated metal-enclosed switchgear (GIS).However, the factors affecting the lightning impulse test waveform are complex and diverse. In this paper, the discharge principle of the impulse voltage generator is studied. Combined with simulation, the influence of different factors on the waveform parameters of lightning impulse test of UHV GIS equipment is obtained. The common methods and improvement measures of waveform debugging are pointed out, and the main factors affecting the parameters of lightning waves are studied.

Non-Linear Insulator Performance of Wind Turbine Blade Surface against Lightning Strike

This research work investigates the surface degradation of blade specimen which is made from epoxy resin mixed with non-linear fillers of Zinc oxide, Aluminium oxide and Sodium silicate in different ratio percentage. Test on surface trackability of each specimen according to IEC 60587 standard. Then, choose the best specimen which has achieved a high tracking resistance to test against lightning impulse voltage according to IEC 60243-3 at ± 60 kVpeak. A comparison has been made with the pure epoxy resin specimen as a controlled sample. The results showed that, the epoxy resin specimen with the ratio of 40% by weight of Aluminium oxide filler slow down the tracking process better than other specimen type. And from the result of lightning impulse withstand voltage test, it is found that all of the specimen were not affected.

FDTD simulation considering an AC operating voltage for air-insulation substation in terms of lightning protective level

The accuracy of lightning overvoltage evaluation of electrical equipment has been improved based on circuit analysis such as the Electro-magnetic Transients Program, but analyzing phenomena whose plane wave propagation (TEM) mode cannot be assumed remains unsolved. Since numerical analysis of electromagnetic fields such as the finitedifference time-domain (FDTD) method directly solves Maxwell’s equations, it is an appropriate means for solving this issue. In this paper, the lightning surge waveform due to the back-flashover is analyzed using the FDTD method; bearing detailed modeling of transmission towers and incoming substation lines in mind. An open end of a circuit breaker of a 500 kV air-insulated substation was selected as the subject of analysis, and ac operation voltage was considered as an analysis condition. Following FDTD analysis, a 150 kA lightning stroke to the first tower, which is the standard lightning stroke of the lightning protection design, does not cause back-flashover, but a 200 kA lightning stroke to the first tower and a 150 kA lightning stroke to the second did do so. The flashover occurred in the later timing than the result of conventional circuit analysis, and the rise of the lightning surge was also slower. The voltage waveform of the open end of the circuit breaker reached its peak after repeating reflections, and the overvoltage value became lower than the circuit analysis. According to the FDTD analysis, the required withstand voltage value after the waveform evaluation with a 200 kA lightning stroke is 1,337 kV, which is lower than the present lightning impulse withstand voltage (LIWV) 1,550 kV. The lightning stroke current 200 kA is the value adopted in the lightning protection design of the UHV system. Therefore, the existing air-insulated substation equipment is considered highly capable of withstanding overvoltage at the time of a back-flashover.

Insulation characteristics of oil-immersed power transformer under lightning impulse and AC superimposed voltage

The insulation performance of an oil-immersed power transformer against lightning surges is verified by applying a lightning impulse (LI) voltage alone. However, the surge voltage generated at the transformer terminal in an actual system is superimposed over the ac operating voltage. Particularly for UHV- and 500 kV-class, where the ratio of ac operating voltage to the LI withstand voltage is high, the insulation characteristics for this LI and ac superimposed voltage are crucial factors to study the insulation design. Accordingly, this paper reviewed the research results of insulation characteristics of transformers for the superimposed voltage. Subsequently, these results were evaluated from perspectives of an insulation coordination and transformer insulation test. The insulation elements of transformers covered were the turn-to-turn and section-to-section insulation, for which the LI test is key for insulation design. As a result, for the section-to-section insulation for a shell-type transformer and the turn-to-turn insulation, the breakdown (BD) voltage for the superimposed voltage remained unchanged from the result when the LI was applied alone. Conversely, for the section-to-section insulation of a continuous disk winding for a coretype transformer, the BD voltage attributable to the superimposed voltage declined by about 10% to 20% from that when the LI was applied alone if the oil-impregnated paper component ratio compared to the oil-gap length was low. This is because the oil gap was primarily responsible for the BD and flaws were generated there when the ac voltage was applied. On the other hand, when this ratio was increased, the oil-impregnated paper part was primarily responsible for BD and the decline in BD voltage could be suppressed. For the section-to-section insulation for the interleaved disk winding, the BD voltage attributable to the superimposed voltage remained constant from that when the LI was applied alone. This is because the BD was determined by the LI partial discharge inception at the turn-to-turn part of the section-to-section insulation, where the superimposed voltage made little impact on BD. Based on the above, the LI test is considered adequate to verify the insulation performance of the section-to-section insulation for a shell-type transformer and for an interleaved disk winding and the turn-to-turn insulation against surge overvoltage. Conversely, for the section-to-section structure of a continuous disk winding, the influence of the ac voltage must be taken into account. These results are key findings for rationalizing transformers while maintaining their insulation reliability.

Insulation characteristics of gas insulated switchgear under lightning impulse and ac superimposed voltage

The insulation performance of gas insulated switchgear (GIS) against lightning surges is verified by applying a lightning impulse (LI) voltage alone. However, the surge voltage generated in an actual system is superimposed over the ac operating voltage to form a waveform that differs from the LI voltage waveform. For UHV- and 500 kV-class GIS in particular, where the ratio of ac operating voltage to the LI withstand voltage is high, the insulation characteristics for this LI and ac superimposed voltage are important factors to study the insulation design. Accordingly, this paper researched the insulation characteristics for the superimposed voltage of element models simulating the SF6 gas gap, creepage surface of the insulating spacer and inside of the insulating spacer, all of which are the main insulating parts of GIS. The ac component ratio of the superimposed voltage ranged from 30 to 70% of that of breakdown (BD) voltage when ac voltage was applied alone to each model, wherein conditions for superimposed phase or similar served as parameters. Subsequently, these results were evaluated from perspectives of an insulation coordination and GIS insulation test. As a result, the BD voltage showed results almost equivalent to those obtained in the case where the LI voltage was applied alone; even when the ac component of superimposed voltage was varied at three different insulation parts. Accordingly, within the 30 to 70% range of superimposed ac voltages, it emerged that the BD voltage was almost determined by the peak value of the applied voltage and was unaffected by the ac component. When comparing with GIS test voltages in the 66 kV to UHV-class, this range of 30 to 70% covers all the ratios of power-frequency and LI test voltages for operating voltages in an actual system. Consequently, the LI voltage test was considered adequate to verify the insulation performance of GIS against surge overvoltage generated during operation whereas superimposition of ac voltage need not be considered. These results are meaningful findings for rationalizing GIS while maintaining its insulation reliability.

Voltage-time Characteristics and related Approximation Formula for 6.6-kV Solid-core-type Insulator having Higher Lightning Impulse Withstand Voltage

Voltage-time Characteristics and related Approximation Formula for 6.6-kV Solid-core-type Insulator having Higher Lightning Impulse Withstand Voltage

Evaluation of overshoot rate of lightning impulse withstand voltage test waveform based on new base curve fitting methods - application to practical diverse waveforms

In a lightning impulse withstand voltage test for large-sized electric power equipment, voltage waveforms sometimes contain overshoot due to the capacitance of the test equipment and the residual inductance of the test circuit. The authors have been studying fitting methods for extracting a reasonable base curve from this overshoot waveform and constructed a new base curve extraction method. In this paper, a verification study was performed using diverse overshoot waveforms to show the rationality of this new method from a more practical perspective. To be specific, the study used actual waveforms with the frequency of the overshoot part varied as well as waveforms with the overshoot rate less than 10%. Consequently, it was found that the new method was a more reasonable one to calculate the overshoot rate of several to several tens of percent which is an actual range for evaluation in the test. Meanwhile, it should be noted that the k-factor filtering scheme was implemented after the application of various fitting methods to test data generator waveforms, and then the shape parameters of the test voltage waveforms, such as the crest value, front time, and time to half-value, were almost identical, which confirmed that fitting methods had only a minor influence on the test voltage waveform. Furthermore, assuming an actual test for UHV-class equipment, the relationship between the front time and the overshoot rate was investigated in detail. As a result, it emerged that, although it was difficult to generate waveforms complying with the present standard if the capacitance of the test equipment was large, the number of equipment that could comply with the standard would significantly increase if the value of standard for the front time was relaxed.

Insulation breakdown characteristics of UHV-class oil-immersed transformer for lightning impulse withstand voltage test waveform - K-factor value and front time related characteristics

The lightning impulse withstand voltage test waveform for electrical power equipment is specified in IEC 60060-1 "High-voltage test techniques". Now that ultrahigh voltage-class (UHV-class) electrical power equipment is being introduced and increasingly used, the electrical capacitance of equipment to be tested has increased, which has eventually hampered the test using the waveform that conforms to the previous standard. To address this issue, the IEC revised the IEC 60060-1 standard in 2010 and now is studying the standard for UHV-class electrical equipment. In this revision, an evaluation method using the k-factor function (test voltage function) was introduced, whereby the overshoot waveform was converted to a test voltage waveform. To date, concerning these k-factor values, measurement results based on experiments have been reported, but most were for small-scale insulation models with breakdown voltage levels mainly around 100 kV. This paper reports the experimental result of insulation breakdown characteristics for lightning impulse withstand voltage test waveforms in the model with the largest oil gap possible assuming an actual UHV-class oil-immersed transformer. Breakdown voltages and breakdown times were measured with the superimposed oscillation frequency, overshoot rate and front time as parameters. Consequently, it was indicated that, as the overshoot rate increased, the 50% breakdown voltage was inclined to increase. Subsequently, the evaluation using the k-factor function is considered to be useful for UHV-class oil immersed transformer. The result of evaluation of the k-factor value based on the experimental result was almost identical to the result of the European Project, which is the basic data of the existing k-factor function. As a result, it emerged that the existing k-factor function itself was most likely to be applicable to UHV-class oil-immersed transformers. Furthermore, since the extension of the front time up to about 3.6 μs caused only a small change in insulation breakdown characteristics, the extension of the front time up to about 3.6 μs might possibly be allowed in the standard that assumes a UHV-class transformer.

Insulation breakdown characteristics of UHV-class gas insulated switchgear for lightning impulse withstand voltage test waveform - K-factor value and front time related characteristics

The lightning impulse withstand voltage test waveform for electric power equipment is specified in IEC 60060-1 "High-voltage test techniques" as revised in 2010. At present, test standards for UHV-class equipment are under study. Increasing equipment capacity and the digitization of measuring equipment are mentioned as these backgrounds. Withstand voltage test by the standard waveform specified in the previous standard had been difficult (it eventually becomes an overshoot waveform) with increasing equipment capacitance. In response, an evaluation method using the kfactor function (test voltage function) was introduced, whereby the overshoot waveform was converted to the test voltage waveform. To date, concerning these kfactor values, measurement results based on experiments have been reported, but most were from small-scale insulation models with breakdown voltage levels mainly around 100 kV. The present study reports the experimental results of the insulation breakdown characteristics for the lightning impulse withstand voltage test waveform in the largest SF6 gas insulation model possible assuming actual UHV-class gas insulated switchgear. Breakdown voltage and breakdown time were measured with the superimposed oscillation frequency, overshoot rate, and front time as parameters. Following evaluation of the k-factor value based on these experimental results, the k-factor value with the overshoot rate of 10% was almost identical to that of the existing k-factor function. Consequently, evaluation using the existing k-factor function is considered appropriate. Subsequently, the existing k-factor function is most likely to be effective also for UHV-class equipment. In addition, it emerged that changes in the insulation breakdown characteristics due to the extension of the front time were small. It was considered that extending the front time, rather than allowing an excessive overshoot rate, would enable proper verification of the insulation performance as part of standard assuming UHV-class equipment.

Evaluation of overshoot rate of lightning impulse withstand voltage test waveform based on new base curve fitting methods - study on overshoot waveform in an actual test circuit

In a lightning impulse withstand voltage test for large-sized electric power equipment, voltage waveforms sometimes contain overshoot due to the capacitance of the test equipment and the residual inductance of the test circuit including the power source. The authors have been studying fitting methods for extracting a reasonable base curve from this overshoot waveform. In the previous study, each of the fitting methods was applied to the overshoot waveform simulated using a mathematical equation to evaluate the respective merits and demerits. As the test waveform, the present paper used the overshoot waveform actually generated using a large-scale test circuit. This actual waveform was determined as one over which an oscillation waveform of several hundred kHz, which was actually at issue, was superimposed. The correct base curve of the actual waveform was derived through detailed analysis of the equivalent circuit of the test circuit and the overshoot rate was calculated based on this correct value. This correct overshoot rate was compared with that calculated using each of the fitting methods to evaluate which was an optimum fitting method. Consequently, it was found that an overshoot rate close to the correct value could be derived using the new base curve extraction method for the actual waveform. Meanwhile, it should be noted that the kfactor filtering scheme was implemented after the application of various fitting methods, and then the shape parameters of the test voltage waveforms, such as the crest value, wavefront duration, and wavetail duration, were almost identical, which confirmed that fitting methods had only a minor influence on the test voltage waveform.

Development of high frequency circuit model for oil-immersed power transformers and its application for lightning surge analysis

The lightning impulse withstand voltage for an oil-immersed power transformer is determined by the value of the lightning surge overvoltage generated at the transformer terminal. This overvoltage value has been conventionally obtained through lightning surge analysis using the electromagnetic transients program (EMTP), where the transformer is often simulated by a single lumped capacitance. However, since high frequency surge overvoltages ranging from several kHz to several MHz are generated in an actual system, a transformer circuit model capable of simulating the range up to this high frequency must be developed for further accurate analysis. In this paper, a high frequency circuit model for an oil-immersed transformer was developed and its validity was verified through comparison with the measurement results on the model winding actually produced. Consequently, it emerged that a high frequency model with three serially connected LC parallel circuits could adequately simulate the impedance characteristics of the winding up to a high frequency range of several MHz. Following lightning surge analysis for a 500 kV substation using this high frequency model, the peak value of the waveform was evaluated as lower than that simulated by conventional lumped capacitance even though the front rising was steeper. This phenomenon can be explained by the charging process of the capacitance circuit inside the transformer. Furthermore, the waveform analyzed by each model was converted into an equivalent standard lightning impulse waveform and the respective peak values were compared. As a result, the peak value obtained by the lumped capacitance simulation was evaluated as relatively higher under the present analysis conditions.

Evaluation of overshoot rate of lightning impulse withstand voltage test waveform based on new base curve fitting methods - study by assuming waveforms generated in an actual test circuit -

The authors have studied fitting methods to extract a more reasonable base curve from the lightning impulse voltage test waveforms. In this paper, by assuming a test circuit of actual large-sized UHV class equipment, simulated recorded waveforms were created, to which various fitting methods were applied in order to study their merits and demerits. Consequently, it was found that a new base curve extraction method could calculate an overshoot rate close to the correct value for waveforms actually generated. In particular, while the existing method resulted in significant dissociation for waveforms superimposed by oscillation waves of several hundred kHz, which was actually at issue, the new method significantly improved the evaluation results of the overshoot rate. Meanwhile, it should be noted that the k-factor filtering scheme was implemented after the application of various fitting methods, and then the shape parameters of the test voltage waveforms, such as the crest value, wavefront duration, and wavetail duration, were almost identical, which confirmed that fitting methods had only a minor influence on the test voltage waveform.

Partial discharge-induced degradation characteristics of insulating structure constituting oil-immersed power transformers

The soundness of an oil-immersed power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. The acceptance criterion for this PD test is 500 pC according to IEC standard; however, only insufficient basic data backing this criterion are available. In addition, the harmful level of the insulation performance has not necessarily been perceived in such a manner as the present one, for example, only the insulation breakdown, instead of the insulation performance degradation, has been assumed to be harmful. In light of the consideration presently paid, it is necessary to determine the PD level that causes the insulation performance to be degraded in order to confirm the soundness by the test using an actual transformer. As the initial step of the study for this purpose, the previous paper investigated the PD and the insulation damage level using oil-impregnated press-boards (PBs) used in the transformer, at which PD is more likely to occur when ac voltage is applied than at any other parts, and evaluated the harmful PD level for insulating materials themselves. To investigate the harmful PD level for the actual transformer, this paper studied the harmful PD level that decreases the residual lightning impulse (LI) withstand voltage using the main insulation model, which is the insulating structure constituting the oil-immersed power transformer. As a result, black discoloration occurred on the surface of the PB and the residual LI withstand voltage also tended to decline when exposed to ac PD of 20,000 pC or more. On the other hand, when it had been exposed to ac PD of 10,000 pC, no visible damage was observed and there was no drop in the residual LI withstand voltage either. Consequently, the harmful ac PD level for the oil-immersed transformer can be determined at between 10,000 pC and 20,000 pC. This result coincided with that for insulating materials themselves, hence it was revealed that the harmful PD level be set to 10,000 pC on a conservative side.