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For ultra‐high‐voltage (UHV) systems together with high‐voltage facilities, more rationalized insulation coordination and equipment design are required. To this end, making large‐scale and long‐term field lightning‐strike observations, large amount of and practical experiments, and introducing new analysis techniques have enabled the development of advanced technologies capable of transforming conventional techniques. This paper reviews what has been developed in a Japanese electric power company in this regard. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

Some cases reported recently indicate flow electrification that becomes obvious due to aging deterioration of insulating oil and cellulose insulation in aged oil-immersed transformers. It is supposed that the inclusion of minor components influences the aging deterioration of mineral insulating oil, but details including the aging mechanism have not been clarified. This paper examines the influence of various compounds on the electrostatic charging tendency (ECT) of insulating oil. Mineral insulating oil mainly consists of hydrocarbon compounds, but it also contains extremely small amounts of sulfur compounds, nitrogen compounds and oxygen compounds. Sample oil was prepared by adding various compounds to synthetic oil suffering from small aging changes of the ECT. After heating the sample oil, the ECT was measured, and the influence of various compounds was examined. Aging changes of the ECTs were small in pure hydrocarbon compounds. On the other hand, increases by aging were detected in the ECTs of the sample oils to which impurities such as sulfur compounds and nitrogen compounds were added. In particular, outstanding increases were detected in the ECTs of sample oil to which sulfide compounds and sulfoxide compounds classified as sulfur compounds were added. It has been estimated that sulfur compounds influence the increase by aging of the ECT of mineral insulating oil. It has been also estimated that the increase by aging of the ECT of mineral insulating oil starts from sulfide compounds contained in new oil and that the ECT increase process triggered by sulfide compounds progresses by the production of sulfoxide compounds and compounds of high ECT. Index Terms--Aging, Copper, Electrostatic measurements, Electrostatic processes, Insulation, Insulators, Nitrogen compounds, Oil insulation, Oxidation, Oxygen, Oxygen compounds, Petroleum, Sulfur, Sulfur compounds, Transformers.

An effective means of rationalization of insulation design for gas insulated switchgear (GIS) is the evaluation of insulation characteristics under actual surge waveforms in the field called non-standard lightning impulse waveforms (non-standard-LIWs). The preceding researches examined quasi-uniform electric field SF6 gas gaps and partly the cone-shaped insulating spacers that represent an insulation element of GIS in order to obtain the insulation characteristics under various non-standard-LIWs. As a result, the breakdown voltages under non-standard-LIWs are higher than those under standard-LIWs, allowing the authors to identify a road map for rationalizing (;IS insulation designs. In applying this rationalization technique, experimental study was conducted on single-frequency oscillations to identify the influence of the non-uniform electric field. As a result, the breakdown characteristics were found to be different from those in a quasi-uniform electric field. This paper describes the insulation characteristics under double-frequency oscillations as typical examples of non-standard-LIWs while varying the applied voltage polarities, frequencies, and damping factors in order to study the influences of non-uniform electric field in further detail. The experimental results show that positive-polarity waveforms pose more demanding insulation requirements and that it is appropriate to conduct experiments using positive-polarity in order to consider dielectric strengths. Moreover, it was confirmed that changing the waveform parameters under positive-polarity did not cause major changes in the breakdown voltages, which remained constantly higher than those under standard-LIWs, and gave little influence to the application of the rationalization techniques. Index Terms--Gas insulated switchgear (GIS), non-standard lightning impulse waveform, dielectric breakdown voltage--time characteristics (V-t characteristics), S[F.sub.6] gas gap, non-uniform electric field, double-frequency oscillations.

As the deregulation of the electric power industry moves forward, there is an increasing need for cost cutting and it is becoming exceedingly important to keep track of the residual life and identify the timing of replacement for power transmission equipment that has aged for several tens of years in the field. This paper, taking up the theme of 'insulation degradation phenomena and lifetime evaluation for power transmission equipment', conducted cross-sectional comparison of different types of equipment although this kind of evaluation was made independently for these equipment types so far. This paper covered six types of equipment: gas insulated switchgear (GIS), gas insulated transformers, oil filled (OF) transformers, OF cables, crosslinked polyethylene (XLPE) cables, and power capacitors. As a result of examining the characteristics such as electric, thermal, mechanical, and hygroscopic degradations, each of these equipment types was found to have distinctive Voltage-time (V-t) and Voltage-number (V-N) characteristics, which sometimes greatly varied from those of others. Since thermal degradation is dominant for gas insulated transformers and oil filled transformers, their lifetimes depend on the reduction of mechanical strengths of Polyethylene Terephthalate (PET) or insulating paper. Therefore, it is effective to diagnose decomposition products such as CO, C[O.sub.2] and furfural generated in the thermal degradation mechanism. On the other hand, the influence of electric degradation has more significant impact on the other equipment types except OF cables. Such GIS components as epoxy spacers, XLPE cables, and power capacitors for which long-term ac voltages are dominant, are commonly characterized by the long-term V-t characteristics with a large gradient (small n-value). In terms of insulation structures, there is a possibility of reducing the degree of degradation by improving solids or the interface between solids and electrodes. Index Terms--Power transmission equipment, insulation degradation, lifetime evaluation, cross-sectional comparison.

To lower the lightning impulse withstand voltage of gas insulated switchgear (GIS) while maintaining the high reliability of its insulation performance, it is important to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the previous work, the dielectric breakdown voltage-time characteristics were measured under several different conditions on the quasi-uniform [SF.sub.6] gas gap and partly the cone-shaped insulating spacers that represent an insulation element of GIS for six kinds of non-standard lightning impulse waveforms associated with lightning surges and disconnector switching surges. This paper, describes detailed examination in non-uniform electric field, and the pattern in which a disconnector switching surges are superposed on residual dc elements generated through the operation of disconnectors (bias voltages). This is intended to simulate more severe conditions in an actual system. Consequently, it was found that the presence of a bias voltage in the nonuniform electric field resulted in lower breakdown voltage values for single-frequency oscillation waveforms both in the positive and negative polarities. However, the breakdown voltage values under positive-polarity waveforms posing more severe insulation conditions were higher than the ones under the standard lightning impulse waveforms. Despite the presence of a bias voltage, it was possible to evaluate these breakdown voltage values in the positive polarity using the concept of a duration examined in the past. Index Terms--Gas insulated switchgear (GIS), non-standard lightning impulse waveform, dielectric breakdown voltage-time characteristics (V-t characteristics), [SF.sub.6] gas gap, non-uniform electric field, bias voltage.

Lightning strokes to transmission lines have been estimated using the electro-geometric model proposed by Armstrong and Whitehead. The observed results of the lightning to large-scale transmission lines are reported here to validate the calculated results, in which ultra high voltage (UHV) designed transmission lines and 500 kV transmission lines were selected as the subjects of the observation. Lightning observations were carried out for direct strokes to phase conductor caused by shielding failures, as well as strokes to ground wires that have rarely been previously reported. The observed results showed that total number of direct lightning strokes to phase conductor were nearly identical to the calculated results based on the conventional method, while observed strokes to upper phase were larger and strokes to lower phase were smaller than those derived from calculations. It was also revealed that lightning strokes to ground wires of UHV designed transmission lines and 500 kV transmission lines were 5.1 times and 2.7 times larger, respectively, than calculated results based on the conventional method. Index Terms--Lightning observation, lightning shielding, direct lightning, UHV designed transmission lines, A-W model, lightning stroke to ground wire.

We have investigated the performance of silicone oil as an insulating medium with the aim of developing environmentally-friendly, flame-retardant electric power equipment. We have studied the streaming electrification characteristics of silicone oil during flow, with regard to its use in the cooling of electric power equipment, at temperatures from 30 to 130 [degrees]C. The effects of the insulator material on the charge density in the oil and on the accumulated charge density in the insulator were investigated. We use four kinds of insulators, including aramid paper, in these experiments. The silicone oil was negatively charged by streaming electrification, in contrast to conventional mineral oil. The magnitude of the accumulated charge density in the insulator was influenced by the kind of insulator. It was also found that the charge density in the silicone oil and the accumulated charge density in the insulator changed with increasing temperature. Moreover, we investigated the effect of the kinematic viscosity on the streaming electrification by performing experiments using oils with kinematic viscosities of 20 cSt and 50 eSt. We have considered the optimum combinations of silicone oil and insulator material. Index Terms--Silicone oil, streaming electrification, charge density, electric power equipment.

The authors have been examining the diagnosis method for partial discharge (PD) occurring inside gas insulated switchgear (GIS) using the UHF method with a focus on electromagnetic (EM) wave mode conversions at L-shaped, T-shaped pipes, and components exsiting in actual GIS. Based on these conversion characteristics, an on-site, high-accuracy PD diagnosis algorithm was constructed for use on high-order mode EM-waves. All the researches so far were targeted at an isolated-phase-type (single-phase-type) tank. The present study was targeted at a straight-pipe three-phase structure, the most basic form, in order to apply the evaluation method to a three-phase-common-type tank (three-phase-type tank). Through the implementation of EM-wave propagation experiments and the EM-field analysis, the presence of TEM, [TE.sub.11], and [TE.sub.21] modes was verified also in the three-phase-type tank. Next, the EM-field analysis was conducted on models equivalent to actual apparatuses of 72 to 550 kV classes. Then, formulas for simplified calculation of cutoff frequencies in [TE.sub.11] and [TE.sub.21] in a three-phase-type tank were proposed. Furthermore, the basic concept of the PD diagnosis method was examined and the work flow was presented. Index Terms--Partial discharge (PD) diagnosis, gas insulated switchgear (GIS), three-phase-type, ultra high frequency (UHF), electromagnetic wave (EM-wave), high order mode.

To lower the lightning impulse withstand voltage of gas insulated switchgear (GIS) while maintaining the high reliability of its insulation performance, it is important to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the preceding researches, lightning surge waveforms and disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations were analyzed and five to six kinds of non-standard lightning impulse waveforms with basic frequencies of 0.6 to 5.0 MHz were identified. Then, the dielectric breakdown voltage--time characteristics were measured under several different conditions on the quasi-uniform S[F.sub.6] gas gaps and partly the cone-shaped insulating spacers that represent insulation elements of GIS for six kinds of non-standard lightning impulse waveforms. In this paper, the resultant breakdown voltages were evaluated in terms of the overvoltage duration, which led to their formulation in a unified way. On the basis of these insulation characteristics and their unified formulation, the paper investigated a method for converting non-standard lightning impulse waveforms into standard lightning impulse waveforms with equivalent stress for the insulation. When the constructed algorithm was applied to five examples of representative two type waveforms in the lightning surge time region, they were converted into standard lightning impulse waveforms with crest values reduced by 20% to 34%, suggesting potentiality for reduction of lightning impulse insulation specifications of GIS. Index Terms--Gas insulated switchgear (GIS), non-standard lightning impulse waveform, waveform conversion method, dielectric breakdown voltage, S[F.sub.6] gas gap.

As a means of diagnosing partial discharge (PD) signals propagate inside a Gas Insulated Switchgear (GIS), a study for the leakage of electromagnetic waves (EM-waves) emitted from the insulating spacer was implemented. The EM-waves leaking out from the solid insulator have the resonance frequencies depend on the spacing between adjacent bolts in the direction of the flange circumference, because the leakage portion is the equivalent of a slot antenna. In this paper, using an electromagnetic analysis model which has a simulated spacer on a concentrically-shaped GIS tank, the output characteristics of the EM-waves that leaked out from the slit were analyzed under various conditions such as the spacing between adjacent bolts, the width of the spacer, the dielectric constant of the spacer and the form of the flange. Also the actual measurement by the experimental equipment used to simulate the model was implemented for comparison with the analytical results. Consequently, the optimal specifications of the sensor and the measurement method used to achieve highly-sensitive detection for practical use were summarized and proposed as well as evaluating the effectiveness of the electromagnetic analysis model adopted in this paper. Index Terms--Gas insulated switchgear (GIS), partial discharge (PD), electromagnetic wave (EM-wave), slot antenna, resonance frequency, highly-sensitive detection.

Field study of lightning stroke effects, particularly the mechanisms that produce distribution line faults, is essential for designing rational lightning protection for distribution lines. The Tokyo Electric Power Company has conducted photographic research and observed voltage and current waveforms due to lightning in actual distribution tines since 1996, obtaining 284 datasets, including 62 for direct hits to the tines. These results permit a comprehensive study of each lightning flash; the photographs show the precise location of the lightning, and the waveforms provide detailed analyses of flashovers in the distribution systems. This paper evaluates typical observed datasets, including surge insulator voltage waveforms. In some cases, the direct lightning strokes did not cause distribution line faults because the power follow current extinguished spontaneously after flashover generation. These results provide a solid basis for examining the mechanisms underlying the production of distribution line faults, and can lead to effective lightning protection practices. Index Terms--Direct lightning stroke, distribution line, flashover, indirect lightning stroke, lightning protection design.

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of oil-immersed transformers and thus reduce equipment cost while maintaining high insulation reliability, it is required to identify the insulation characteristics under non-standard lightning impulse waveforms that are associated with actual surge waveforms in the field and quantitatively compare them with the characteristics under the standard lightning impulse waveform. In the previous research, field overvoltages in the lightning surge time region were analyzed, and four typical non-standard lightning impulse waveforms were defined. These four waveforms were used to measure the breakdown voltages and the partial discharge inception voltages on three models of the winding insulation elements of oil-immersed transformers. The average breakdown voltages were evaluated in terms of the overvoltage duration. This paper describes a method for converting of non-standard lightning impulse waveforms into standard lightning impulse waveforms with equivalent stress for the insulation. The constructed algorithm was applied to four examples representing two types of non-standard lightning waveforms. Due to the conversion into standard lightning impulse waveforms, the crest values were reduced by 14% to 26%. This seems to be a potential for reduction of lightning impulse insulation specifications of oil-immersed transformers. Index Terms--Oil-immersed transformer, non-standard lightning impulse waveform, waveform conversion method, dielectric breakdown voltage, partial discharge inception voltage, oil-gap, turn-to-turn insulation model, section-to-section insulation model, lightning impulse withstand voltage.

Reliability evaluation method with Weibull distribution function is used for development of test conditions for short-duration power frequency withstand voltage test. Although the evaluations by 'independent method' and 'cumulative method' are proposed, the handling of multiple voltage application still remains as one of the subjects. Therefore, insulation test with one-minute step-up method is studied as an insulation test with coexistence of multiple voltage level. As a result, the insulation characteristics by one-minute step-up method deviated from their true values, in this paper, it was calculated and evaluated by two methods with parameters of test condition. For the one-minute step-up method, the deviations from the true values were found to be influenced by test conditions: the larger the test voltage step width, or the lower the test starting voltage, or the greater the number of voltage steps, the smaller the deviations from the true values. Furthermore, the approaches to designing appropriate conditions for the test were summarized in order to design test conditions for oil-filled transformer and gas insulated switchgear. As a result, the reasonable test conditions were evaluated as follows: for oil-filled transformers, the test starting voltage was 70% of the one-minute breakdown voltage value and the voltage step was 7% and, for gas insulated switchgear, the test starting voltage was 80% and the voltage step was 5%. This result is approximately the same as the test conditions used for the actual measurement data in the past. These examinations will help to establish appropriate test conditions in the one-minute step-up method and to evaluate insulation characteristics obtained from the test. Index Terms--Power-frequency withstand voltage test, one-minute step-up method, test starting voltage, voltage step, reliability evaluation, Weibull distributions, breakdown voltage-time characteristics.

Because of the excellent high insulation performance, vacuum is expected to be one of the alternatives of [SF.sub.6] gas. In order to reduce the use of [SF.sub.6] gas in substation equipment, the applicability of vacuum circuit breakers (VCB) and vacuum interrupters (VI) should be extended to higher (typically 145-300 kV) voltage. For the high voltage VI, the role of electrical insulation design becomes more important. For accurate insulation design of the VI, we developed an optimization technique to improve the electrical insulation performance. We evaluated several design variables necessary, for optimizing the electrode contour of the main contactor and center shield in VI. The electrode area effect on vacuum breakdown characteristics as well as the electric field distribution was considered in the optimization process. In order to verify the accuracy and the efficiency of the proposed technique, we applied it to a practical model of VI. In addition, we examined the influence of the parameters of the area effect, arrangement of VI in VCB on the optimization result. From the calculation results, we confirmed that the optimization based on the area effect had higher availability and higher accuracy than the electric field optimization from the viewpoint of the improvement of the electrical insulation performance of vacuum insulated equipment. Index Terms--Numerical optimization, vacuum interrupter (VI), vacuum insulation, area effect, center shield electrode.

The leakage of electromagnetic waves and high frequency surges caused by the partial discharge inside gas insulated switchgear (GIS) to the outside of its tank may become a signal for the partial discharge detection, and, on the other hand, possibly cause electromagnetic interference (EMI) to electronic equipment via the grounded system. It is, therefore, important to investigate and understand the leakage phenomenon of electromagnetic waves and high frequency surges to the grounded system of the GIS. In the present study, experiments of the characteristics of leaking electromagnetic waves and high frequency surges were conducted using test models for the cable connections, the disconnecting switch (DS), the earth switch (ES), the capacity type potential divider (PD) inside the GIS, and the grounding wire to clarify the characteristics of the respective elements. These results could be approximated using distributed parameter circuits, and models for the leakage surge analysis were proposed. Index Terms--Gas insulated switchgear (GIS), leaking electromagnetic waves, high frequency surges, cable connections, disconnecting switch (DS), earth switch (ES), capacity type potential divider (PD), grounding wire, framework, EMI.

To lower the lightning impulse withstand voltage of gas insulated switchgear (GIS) while maintaining the high reliability of its insulation performance, it is important to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the preceding researches, the dielectric breakdown voltage--time characteristics were measured under several different conditions on the quasi-uniform [SF.sub.6] gas gap and partly the cone-shaped insulating spacers that represent an insulation element of GIS for six kinds of non-standard lightning impulse waveforms associated with lightning surges and disconnector switching surges. As a result, in the tested range, the dielectric breakdown values for non-standard lightning impulse waveforms were higher than those for the standard lightning impulse waveform by 3% to 36%. In these experiments, parameters other than waveforms were fixed to standard conditions. In this paper insulation characteristics on quasi-uniform [SF.sub.6] gas gaps were examined while changing conditions on gas pressures, gap lengths, electrode surface roughness, voltage polarities, and bias voltages. As a result, it was confirmed that the results under standard experimental conditions in the preceding experiments can be applied widely to the GIS actual equipment conditions. Index Terms--Non-standard lightning impulse waveform, gas insulated switchgear (GIS), dielectric breakdown voltage--time characteristics (V-t characteristics), gas pressure, gap length, electrode surface roughness, bias voltage.

For the development of test conditions for a short-duration power-frequency withstand voltage test, which is one of the insulation specifications of substation equipment, there is a precise method of evaluating reliability using a Weibull distribution function. For this method, 'independence method' and 'accumulation method' have been proposed in order to handle the coexistence of multiple voltage levels. In this paper, an insulation test based on the one-minute step-up method as condition for the coexistence of multiple voltage levels is used to calculate and evaluate the test results using these two methods. Consequently, the insulation characteristics obtained from the one-minute step-up test are found to have differences from the true values, if the independence method is adopted, there is a certain deviation regardless of the Weibull time shape parameter 'a-value' and, the smaller the Weibull voltage shape parameter 'm-value', the larger the deviation, if the accumulation method is adopted, on the other hand, the larger the 'a-value' or the smaller the 'm-value', the larger the deviation. The 50% breakdown voltage value, the standard deviation '[sigma]-value', and the 'm-value' that can be obtained from the insulation test based on the one-minute step-up method were evaluated for their deviations from the true values. The results suggest that, when a test is conducted on an oil-filled transformer, the design may turn out to be on the dangerous side due to overestimation of the dielectric strength by around 8%. Index Terms--Power-frequency withstand voltage test, one minute step-up method, reliability evaluation, Weibull distributions, breakdown voltage-time characteristics.

The diagnosis algorithm using ultra high frequency (UHF) method with the higher-order electromagnetic wave (EM-wave) was examined as the advanced partial discharge (PD) diagnosis method in a gas insulated switchgear (GIS) in the former paper. Previous studies evaluated the characteristics of EM-waves propagated in a simple coaxial cylindrical pipe, or through an L-shaped or a T-shaped pipe formed by combining these. In the present study, components (a hand hole, a post-type spacer, etc.) present in an actual GIS structure were identified, and an evaluation done mainly through an analysis on what effects their presence has on the mode transformation of EM-waves propagated inside a GIS tank. The direction in which a GIS tank is bent is horizontal (0[degrees]) in the L-shape structure. If there is either a post-type spacer or a shield ring inside, and the signal source is in the 0[degrees] direction, the distribution of the EM-wave strength varied according to through the L-shape structure. Also in the case of a T-shaped structure, the distribution of the output after signals are transmitted through the T-shaped junction varied depending on each component. Based on these characteristics, the diagnosis algorithm for PD with higher-order mode EM-waves was improved taking into account the effects of various components existing in an actual GIS, and a PD diagnosis method more suitable for an actual structure was established. Index Terms--Partial discharge (PD), UHF, diagnosis, electromagnetic wave (EM-wave), higher-order mode, actual GIS structure

For the development of a 'short-duration ac withstand voltage test', an insulation specification of substation equipment, there is a precise method of reliability evaluation using a Weibull distribution function. Regarding this method, there remains a subject of handling coexistence of multiple voltage levels. This paper first defines the two reliability evaluation methods, 'independence method' and 'accumulation method', applying to Weibull evaluation for coexistence of multiple voltage levels in relation to their physical meanings. Next, the influence of the Weibull parameter values are examined on the cumulative fault probabilities and test voltages calculated using these methods. When the time shape parameter a>1, the accumulation method gives higher values than the independence method; When a=1, the two methods give the same values; When a Index Terms--Short-duration ac (power-frequency) withstand voltage test, temporary overvoltage, Weibull distribution, reliability evaluation, cumulative fault probability, substation equipment.

The features of lightning shielding of transmission lines have been calculated using the electrogeometric model (EGM) proposed by Armstrong and Whitehead. Discrepancies, however, have been found in the case of ultra-high voltage (UHV)-designed transmission lines. UHV-designed transmission lines often pass through high-altitude mountain areas, where foggy or rainy conditions are common when there is lightning. While the electrogeometric model determines the striking distance based on the lightning stroke current, environmental conditions, and how they might affect lightning discharge paths, they are not considered. It is possible, therefore, that foggy conditions may cause differences in the performance of transmission-line lightning shielding. This study used scale models of transmission lines for discharge tests with a 1-m air gap and analyzed differences in discharge paths and locations in dry and foggy conditions. The results showed that the polarity of the discharge had a significant influence, but differences in dry and foggy tests were negligible. Index Terms--Discharge characteristics, foggy conditions, lightning shielding, ultra-high voltage (UHV)-designed transmission lines.

The Tokyo Electric Power Company had conducted field research of voltages and currents in actual distribution lines due to lightning strokes between 1997 and 2001. These results confirm that the distribution line faults are mainly caused by direct lightning strokes to the line. Thus, it is important to simulate lightning performance for direct strokes to establish a lightning protection design of distribution lines. In this paper, statistical data of the observed voltages and currents are analyzed, and the validation of the observation results for the direct lightning strokes using a lightning surge analysis model is conducted. The observation results and the simulation model are useful in clarifying lightning performance in distribution lines. Index Terms--Direct lightning stroke, distribution line, lightning current, lightning protection design, lightning surge analysis model, soil ionization.

Currently, environmental problems such as global warming are important issues, and [SF.sub.6] has been identified as a greenhouse gas with a long atmospheric lifetime. Therefore, in the long term, it is preferable to reduce the amount of [SF.sub.6] used as an insulating gas. It is thus important to discuss the possibility of using more environmentally friendly gases as alternative insulation for gas-insulated apparatus. In this paper, we describe the fundamental insulation breakdown characteristics of high-pressure C[O.sub.2] gas at gas pressures of up to 1.0 MPa under simulated practical conditions, including the insulation breakdown characteristics of a high-voltage conductor with an insulating coating and the effect of decomposition products on the creeping insulation of a spacer. With the aim of enhancing insulation performance, we discuss the effect of the type of insulating coating on insulation performance. The breakdown electric field was increased by 20% by coating the conductor. It was verified that the application of an insulating coating is a practical method for enhancing the insulation performance of high-pressure C[O.sub.2] gas. It was also verified that the decomposition products have only a slight effect on the creeping insulation of the spacer except for when there is heavy pollution on the insulating spacer. However, if a large amount of decomposition products is expected to be deposited on the insulating spacer during operation, which may cause a severe interruption to the current, it will be necessary to consider this factor in the insulation design. Index Terms--Insulating gas, [SF.sub.6] substitute, C[O.sub.2] gas, gas-insulated switchgear (GIS), insulation breakdown characteristics, electrode coating, decomposition products.

Since the majority of vacuum circuit breakers in use are of the single-break variety, there have been few studies of double-break vacuum circuit breakers. This report describes our investigations of the interruption characteristics of the double-break vacuum circuit breaker. In this investigation, the post arc current and recovery voltage distribution ratio were measured in order to clarify the relationship between them. The results show that a bias of the voltage distribution ratio occurs when the post arc current value becomes imbalanced. In addition, the measurements of the post arc current for circuit breakers with spiral contacts are very scattered as compared with those with axial magnetic field contacts. Normally, the double-break vacuum circuit breaker consists of two contacts of the same type. However, when two spiral contacts are used, the scattering of post arc current causes a current imbalance. As a result, a bias of the voltage distribution ratio may occur. It appears to be preferable to use double-break vacuum circuit breakers with axial magnetic field contacts. Index Terms--Double-Break, grading capacitors, post arc current, vacuum circuit breakers, vacuum interrupters, voltage distribution ratio.

The electric geometry model suggested by Armstrong and Whitehead was used to calculate the performance of lightning-shielded transmission lines. However, since the ultra high voltage (UHV) designed transmission lines were brought into 500 kV operation, it has become evident that their actual performance is different from the calculated predictions in terms of their actual faults, lightning observation results, etc. It is thought that the UHV designed transmission lines are subjected to foggy or rainy conditions when lightning strikes, because they often pass through mountainous areas at higher altitudes. However, according to the electric geometry model, the striking distance is determined by the lightning-stroke current value only, and the model does not consider the environment of discharge path for the relevant lightning strokes. These considerations suggest that foggy conditions may affect transmission line lightning shielding performance. In this study, discharge tests were conducted using a scaled-down transmission line with a 5 m air gap. The discharge point to the simulated conductor was analyzed in both dry and foggy conditions. Tests were also conducted with a dc bias applied to the simulated power lines, to take the operating voltage of the lines into consideration. These experiments have shown that the effect of fog on the discharge ratio to the conductors is negligible. Index Terms--Lightning, shielding, discharge characteristics, fog, long air gap, switching impulse.

High frequency surges leaking through the insulated joint of a GIS were measured, using a model GIS, and the phenomena, characteristics and mechanism were clarified based on the experiment and the circuit theory. High frequency surges decrease the impedance of the insulated joint, producing an almost short circuit state, and the coefficient for refraction from the inside of the GIS tank to the grounding system becomes significantly small. In a multi-phase GIS, surges leaking through the insulated joint form the tank-to-tank mode voltage for the different phase tank, generating the voltage equal to the potential of the voltage application tank to ground. On the other hand, no potential to ground is directly generated to the adjoining bus. The amplitude and the frequency of surges leaking through the insulated joint of a GIS bus are almost identical to those calculated from the distributed constant circuit theory. The phenomena can be, therefore, reproduced using the EMTP through the simulation of the line part by the multi-phase distributed constant and the simulation of the insulated joint by the capacitance. Index Terms--Gas insulated switchgear (GIS), leaking electromagnetic waves, high frequency surges, insulated joint, EMI, EMTP.

This paper describes discharge properties of [N.sub.2] and C[O.sub.2]-based gas mixtures including a perfluorocarbon (PFC) gas such as [CF.sub.4], [C.sub.3][F.sub.8] and c-[C.sub.4][F.sub.8] under non-uniform field. The mixture ratio between a base gas of [N.sub.2] or C[O.sub.2] and the additive PFC gas was fixed as 9:1; namely, 90%[N.sub.2]/10%PFC or 90%C[O.sub.2]/10%PFC gas mixture. The PFC gases have even smaller global warming potential (GWP) than [SF.sub.6] gas and have good insulation properties as [SF.sub.6] gas. Thus, PFC gas mixture is expected to be a [SF.sub.6] substitute without highly pressurizing the gas over the conventional pressure of 0.5 to 0.6 MPa. In this study, in order to compare the partial discharge (PD) inception voltage [V.sub.PD] and breakdown voltage [V.sub.B] properties between [N.sub.2] and C[O.sub.2]-based gas mixtures, as well as between the additive gas of PFC and [SF.sub.6] gas, we investigated these properties of the gas mixtures with a needle to plane electrode under ac high voltage application. The gas pressure was changed from 0.1 to 0.6 MPa. As a result, it was found that [V.sub.PD] and [V.sub.B] characteristics of [N.sub.2] and C[O.sub.2]-based gas mixtures differed considerably, especially the gas pressure dependence of [V.sub.B] (so-called the N shape characteristics). [V.sub.B] characteristics of [N.sub.2]-based gas mixture including c-[C.sub.4][F.sub.8] proved to be excellent within the test conditions over the wide gas pressure region, showing the maximum breakdown voltage. In terms of [V.sub.PD] properties, C[O.sub.2]-based gas mixture had an advantage over [N.sub.2]-based gas mixture due to higher [V.sub.PD]. Furthermore, we discussed the synergy effects of [V.sub.PD] and [V.sub.B] for [N.sub.2] and C[O.sub.2]-based gas mixtures using the index [R.sub.n] which was defined to quantify the degree of the effect. [R.sub.n] for C[O.sub.2]-based gas mixture was higher than that of [N.sub.2]-based gas mixture. Index Terms--[SF.sub.6] gas, perfluorocarbon (PFC) gas, partial discharge, breakdown voltage, non-uniform field.

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of a gas insulated switchgear (GIS) and thus cut the equipment cost while maintaining the high reliability of its insulation performance, it is necessary to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the preceding paper, lightning surge waveforms and disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations were analyzed and five to six non-standard lightning surge waveforms with basic frequencies of 0.6 to 5.0 MHz were identified. In this paper, the dielectric breakdown voltage--time characteristics were measured under several different conditions for the quasi-uniform [SF.sub.6] gas gap that represents an insulation element of a GIS toward two kinds of non-standard lightning impulse waveforms associated with disconnector switching surges. As a result, in the tested range, the dielectric breakdown values for non-standard lightning impulse waveforms were higher than for the standard lightning impulse waveform by 6% to 36%. Index Terms--Non-standard lightning impulse voltage waveform, disconnector switching surge, gas insulated switchgear (GIS), dielectric breakdown voltage--time characteristics (V-t characteristics), lightning impulse withstand voltage (LIWV).

Gas insulated Switchgears (GIS) are widely used as major substation equipment because of their excellent insulation and interruption performance. However, while a GIS is highly reliable, it is a concealed device, meaning diagnostic technology using various sensors is required to detect any insulation abnormality. Among various potential defects inside a GIS, the most problematic is caused by the existence of fine metallic particles, and thus the ability to detect harmful particles in tests at a factory and on site as well as in operation is becoming an important subject. This paper examines, on an experimental basis, the UHF sensor's characteristics of detecting partial discharge (PD) signals generated from particles inside the GIS with those placed freely on the bottom surface of the tank, having adhered to the insulating spacer's surface, and fixed on the center conductor, respectively. Based on previous reports, breakdown characteristics with the existence of particles were arranged in terms of the particle length, and the harmful particle length was estimated. Both results were compared with each other to evaluate the possibility of detecting harmful particles in each location with the UHF sensor. Consequently, it emerged that free particles were detected comparatively easily. However, while those having adhered to the spacer's surface and fixed on the center conductor could be detected in insulation tests at 2 pu at a factory, detection was difficult in many cases in the field. Index Terms--Gas insulated switchgear, GIS, metallic particle, harmful particle length, partial discharge, PD, UHF sensor.

The electrogeometric model has been used to calculate the transmission line lightning shielding characteristics. However, actual UHV design transmission line accidents and the lightning observation results indicate that there are differences between what actually happens and theoretical results. In this paper, long-gap discharge tests were conducted to better understand lightning shielding characteristics, mainly of UHV design transmission lines. As far as the test method is concerned, the arrangements of transmission line conductors were reduced, and simulated as ground electrodes, and a switching impulse voltage was applied to rod electrodes. As a result, discharges occurred most frequently to the conductors with the smallest gaps with the rod electrodes. On the other hand, it was seen that the electric field strength of the transmission lines and their position with respect to ground might have some influence. These results might also be valuable for long air gap dielectrics. Index Terms--Long air gap, discharge characteristics, switching impulse, lightning shielding, EGM.

Dielectric withstand voltage tests to verify oil-filled transformer insulation performance are in most cases carried out using two to three types of tests from lightning impulse (LI), switching impulse (SI) and power frequency (PF) withstand voltage tests based on economical factors and past achievements. Test conversion factors (TCF) are used to evaluate whether the tests cover overvoltages not directly dealt with in tests and the omitted tests. In this paper, insulation characteristics for LI, SI, and PF (ac) are first organized including the recent ones. Several sets of TCF are next calculated based on these characteristics, compared with TCF in present standards, and representative values are then derived. Finally, the dielectric withstand voltage tests specified in present standards are compared using the representative TCF, and the ac withstand voltage test was found to be the severest test overall. Index Terms--Dielectric withstand voltage test, test conversion factor (TCF), oilfilled transformer, lightning impulse withstand voltage (LIWV), switching impulse withstand voltage (SIWV), power frequency withstand voltage (PFWV, ACWV).