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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]

To study the dc insulation design of gas insulated switchgear (GIS), the insulation characteristics under lightning impulse (LI) voltage with a superimposed dc voltage (superimposed voltage) must be clarified. The paper experimentally examined the GIS breakdown characteristics under this superimposed voltage. The test models simulating an insulator creepage surface were used for which consideration of the influence of dc voltage among various other GIS insulating elements is particularly important. To be specific, a cylindrical model made of epoxy resin or fiber-reinforced plastic (FRP) as the material and a conical epoxy spacer model were tested. For the cylindrical model, a cap-shaped electrode was placed on the insulator and a small gap was established between the end of the electrode and the insulator. When the dc breakdown voltages were measured using these samples, they were higher for the applied voltage of positive polarity than that of negative polarity for all samples. The post-test observation of the electrification condition revealed greater electrification on the insulator surface for the applied voltage of positive polarity. The electrification charges are considered to have relaxed the electric field and increased the breakdown voltage. Subsequently, the breakdown test was conducted using a superimposed voltage, whereby a foregoing dc voltage was applied to samples for a certain period, whereupon a LI voltage was applied with the dc voltage continually applied. The breakdown voltage when the LI voltage and dc voltage had equivalent polarity was approximately same to the LI alone breakdown voltage. Conversely, when they were opposite in polarity, the breakdown voltage under the superimposed voltage obviously tended to decrease from the LI alone breakdown voltage. It is considered attributable to the fact that the insulator surface was electrified by the foregoing dc voltage and applying LI voltage opposite in polarity to this electrification intensified the electric field where the breakdown started to occur. Accordingly, it emerged that the GIS breakdown characteristics changed significantly depending on the polarity combinations of LI and dc voltages. The influence of these polarities must be taken into consideration when studying the GIS dc insulation design. [ABSTRACT FROM AUTHOR]

The paper investigated the behavior and partial discharge (PD) characteristics of a metallic particle under residual dc voltage, using the setting conditions and length of the particle, the applied dc voltage, and other factors as parameters and a gas insulated switchgear (GIS) bus bar model equivalent to that of an actual 300 kV GIS. A metallic particle repeated reciprocating movement at a relatively high frequency of about 5 times per second between the high-voltage conductor and the sheath when the electric field at the bottom surface of the tank exceeded its lift-off electric field. It emerged that, in the process of this reciprocating movement, PD occurred at the moment when the particle collided with the high-voltage conductor. In addition, when a particle collided with the electrode, the electrical charges moved and the residual dc voltage was damped. Conversely, where the sheath side was insulation-coated, the particle did not move at the normal operating voltage level, and even though it moved if vibration was applied, its movement stopped relatively soon. If a metallic particle exists in GIS and moves, there is concern that the insulating performance may decline significantly. When a metallic particle moves under dc voltage, a relatively stable PD is generated and consequently, for example, the PD measurement is considered an effective way to detect a particle. It is also considered effective to take physical measures, such as using an insulating sheath at the bottom surface of the tank to restrain the behavior of particles. [ABSTRACT FROM AUTHOR]

Now that gas insulated switchgear (GIS) for ac systems is becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. The insulating portions for which the influence of dc voltage must be taken into consideration are solid insulators, such as insulating spacers. Under dc voltage, since the electric field distribution in an insulator differs from that under ac or impulse voltage and is governed by the resistance characteristics, clarifying its characteristics is crucial to study the GIS dc insulation design. As a solid insulator, focusing on fiber-reinforced plastics (FRP) used for GIS, for example, insulating rods, as well as partially treated epoxy resin; this paper experimentally investigated the bulk and surface electric conductivity under dc voltage, using the electric field, temperature, and other factors as parameters. As a result, the bulk electric conductivity of FRP in an edgewise direction exceeded that in the penetrating direction by one digit. It emerged that the electric conductivity of an insulating material with orientation like FRP varied depending on its direction. It was also found that, despite the fact the bulk and surface conductivity depended on the electric field for both FRP and epoxy resin, the variation width was relatively narrow within the range of the actual GIS operating electric field. The bulk and surface electric conductivity were also temperature-dependent, which meant the variation width was relatively wide. Furthermore, the surface electric conductivity was measured in SF6 gas and in the air to investigate the influence of the ambient atmosphere, whereupon it emerged that the electric conductivity was higher in air due to the adherence of moisture. As mentioned above, the electric conductivity of an insulator varies due to various factors, such as the influence of the material orientation, electric field, temperature, and moisture. Consequently, the electric field distribution inside the insulator also changes, meaning these electric conductivity characteristics must be taken into consideration to study the GIS dc insulation characteristics. [ABSTRACT FROM PUBLISHER]

Thanks to its excellent insulation and cutoff performances, SF6 gas has been applied to power equipment since the 1960s and is now widely used for several kV to 1,000 kV-class GIS, GCB, and GIL. However, since 1997, when SF6 was designated at COP3 as a greenhouse gas to be reduced, there has been a wish to use an alternative insulating gas to pure SF6 gas. In the present study, alternative gases were selected from among mixtures excluding SF6 with the need to reduce GWP (global warming potential) in mind. Gas mixtures containing such substances and with a boiling point of -20°C or less, chemically stable, non-toxic, and not ozone-depleting were prioritized. Furthermore, the availability and environmental performance were taken into consideration when deciding on component gases. Consequently, to launch this series of studies, four types of gas mixtures were used combining a gas in Group A (C2F6, C3F8) ? electronegative gases with relatively high dielectric strength ? and a gas in Group B (N2, CO2) ? gases existing in the natural world. The GWP of SF6 is 22,800 whereas that of C2F6 is 12,200 and that of C3F8 is 8,830, or several times smaller than that of SF6. In the present paper, insulation characteristics were experimentally obtained while varying the mixture ratio under a quasi-uniform electric field assuming GIS. Consequently, compared to the GWP of pure SF6, the GWP was about 12% to 38% for gas mixtures with C3F8/N2 or C3F8/CO2 and 18% to 70% for gas mixtures with C2F6/N2 or C2F6/CO2. Consequently, it emerged that, while assuming breakdown voltage proportional to gas pressure, the GWP was likely to be reduced by 30% to 90% while maintaining dielectric strength. In addition, a study was conducted on the synergism of a gas mixture through analysis using the Boltzmann equation. Consequently, the synergism was confirmed while its degree varied depending on the type of each gas mixture, and the mechanism thereof was clarified. [ABSTRACT FROM PUBLISHER]

Now that gas-insulated switchgear (GIS) for ac systems are becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. For GIS insulating epoxy spacer, the present paper experimentally and analytically studied the influence of spacer surface condition on dc withstand voltage performance. In specific terms, initially, an insulating spacer model with an area of non-uniform resistivity present in the surface layer was created and the dc voltage breakdown characteristics were obtained. As a result, it was clarified that the time delay to breakdown increased with lowering the applied voltage, or in other words, even if a low voltage was applied, breakdown may occur after an extended period. Subsequently, using the same non-uniform resistivity model, the change in the electric field distribution over time under dc voltage was investigated through transient electric field analysis. Consequently, it was found that the electric field distribution varied from a capacitive to a resistive field and the maximum electric field was generated in the boundary between the high- and low-resistivity areas. It was further clarified, based on these breakdown characteristics and electric field analytical results, that the presence of an area of nonuniform resistivity on the insulating spacer creepage surface caused electric field concentration over time and breakdown occurred if the electric field in the creepage surface direction exceeded 40 kV/mm or so. Moreover, the breakdown characteristics were investigated where a lightning impulse voltage was superimposed over a dc voltage. Consequently, the creepage breakdown electric field where a dc voltage was applied alone was almost identical to that where the superimposed voltage was applied. It means that the application of the superimposed voltage may potentially allow the insulating spacer withstand voltage performance to be evaluated by a shorter time test for a dc voltage alone or the dc voltage with a superimposed VFTO. [ABSTRACT FROM AUTHOR]

The IEC-60060-1 "High-voltage test technique" was revised in 2010 and an evaluation method using the k-factor function (test voltage function) was introduced for the lightning impulse voltage test, which involved the overshoot waveform being converted into the test voltage waveform. Test standards for UHV-class equipment are also presently being studied. To date, authors have measured k-factor values with the largest possible model, assuming actual UHV-class power equipment and proposed a new k-factor function for large-scale equipment. The present paper reports on the evaluation results of expanded uncertainty when measuring a k-factor value on the basis of the ISO/IEC Guide 98-3. As a result, it was clarified that significant uncertainty in measuring a k-factor value is brought about by the definition of the kfactor itself because small differences in the 50% breakdown voltage between the smooth lightning impulse and overshoot waveforms with respective uncertainties at a certain level were calculated. The expanded uncertainty when measuring the k-factor value in the present study for an overshoot rate of 10%, which is particularly important for practical use, was a large value of ?0.23 to ?0.36, or 0.46 to 0.72 in topto- bottom width compared to k-factor values themselves ranging from 0.2 to 1.0. However, this is actually considered a minimum level of uncertainty, which is inevitable in principle, and the best achievable measurement result rather than something caused by any problem in measurement itself. [ABSTRACT FROM AUTHOR]

Gas insulated switchgears (GIS) have been increasingly introduced as main substation equipment since the late 1960s thanks to their high reliability, safety, and compactness. The UHF method to diagnose partial discharge (PD) is broadly employed in the field as an advanced insulation diagnostic technology. There are three primary factors related to the PD signal detection characteristics using this UHF method; namely the frequency spectrum of the PD signal itself inside GIS, the propagation characteristics of the PD signal inside the GIS tank, and the detection characteristics of the sensor measuring the PD signal. Accordingly, to further improvement of the diagnostic technique based on the UHF method, the PD phenomenon itself and its propagation characteristics should be clarified and the measurement system refined. From these perspectives, the present study reviewed the recent and latest findings concerning the PD phenomenon and its propagation characteristics inside GIS as well as the PD measurement system verification methods and diagnostic technologies, and summarized typical examples. Firstly, with regard to the PD phenomenon, measurement up to the high frequency band exceeding 10 GHz (even 30 GHz) clarified that the rise time of the PD current waveform is several tens of picoseconds, shorter than previously known. In the case of PD in micro-defects inside the epoxy insulator, however, the rise time is relatively long, in the order of nanoseconds, and a crack is the most critical defect. Subsequently, with regard to UHF PD diagnostics/monitoring technology, a novel technique using PD current waveform characteristics has been developed, and a more advanced PD diagnostic algorithm has been established by clarifying the influence of the shape and GIS internal structures on the electromagnetic wave propagation characteristics. Less uncertain and simplified calibration and verification technologies are also proposed both for a single UHF sensor and the entire UHF measurement system. These new technologies and further advanced studies in future are expected to make the UHF method more convenient and sophisticated. [ABSTRACT FROM AUTHOR]

Now that gas insulated switchgear (GIS) for ac systems are becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. In this paper, to understand the insulation characteristics of epoxy resin, which is widely used for GIS insulating spacers, factors determining the resistivity of the epoxy insulator surface layer under dc voltage were initially investigated on an experimental basis. Consequently, it emerged that the bulk resistance was more dominant than the surface resistance for the dc resistance of epoxy resin due to the dependency of the test sample resistance value on their radius. Since the electric field might be concentrated if some part of this insulator surface layer showed non-uniform resistivity, the influence of the curing agent, one of the potential causes of this non-uniformity, was subsequently investigated with its content as a parameter. As a result, the volume resistivity in the long-term region was likely to decline or vary for epoxy resin containing less curing agent due to the presence of numerous polarized components unreacted with curing agent. In addition, the presence of micro protrusions or similar, if any, on the insulator surface or electrode is considered to cause electrification due to the concentration of electric field on the surface layer. Accordingly, the relationship between their surface roughness and electrification level was investigated using gaps between insulators or an electrode and an insulator facing each other, respectively. Consequently, where the surface roughness of the insulator or electrode was high, a current component with a large damping time constant, considered attributable to electrical charges moving across the gap, appeared after the charging current components and an electrification condition was observed. [ABSTRACT FROM AUTHOR]

The paper focuses on the factors that lower the dielectric performances of real-size gas insulated systems, and show the examples of the measures by using dielectric coatings. The factors studied in this paper are area effects of electrodes and metallic particle motion which are now dominant to determine the size of gas insulated switchgear (GIS). The gases for the systems are basically SF6 but potential gases like N2/SF6 gas mixtures, CO2, dry air and N2 are also included. Area effects of electrodes are studied by reviewing the minimum breakdown data at lightning impulse (LI) of large effective electrode area over about 5 x 104 mm2 under high gas pressures up to 1.9 MPa. The breakdown electric field strength (BDE) or breakdown voltage (BDV) is normalized by the theoretical value and evaluated in terms of effective electrode area and gas pressure. Under the same gas pressure in the range between 0.2 and 0.7 MPa at negative LI, the normalized values of SF6 are lower than those of the potential gases which show similar values. However, the normalized BDVs of N2 at positive LI are further lower than those of SF6 at negative LI. The advanced dielectric coatings enable high dielectric performances over 80% of the theoretical values in SF6 and CO2. Concerning the second factor, metallic particle motion, the experimental data of metallic particle levitation and crossing to high voltage electrode are gathered for dielectric coated and bare enclosures in SF6, N2/SF6> and dry air. The data are classified into two cases of with and without mechanical impulsive force to the enclosure, which simulates the force by the operation of circuit breakers, disconnecting switches and earthing switches. The particle lift-off/levitation electric field strength on the inner surface of dielectric coated enclosure is about 2 to 11 times higher than that on bare enclosure without a mechanical vibration. With a mechanical vibration, the particle lift-off/levitation and crossing electric field strength on dielectric coated enclosure decrease but still about 2 to 3.5 times and 1.3 to 1.6 times higher than those on bare enclosure, respectively. [ABSTRACT FROM AUTHOR]

To ensure the reliability of oil-immersed power transformers in long-term operation, the influence of aging on various characteristics of their insulating elements must be clarified. The present paper reviewed the degradation status of insulating oil and paper of field-aged transformers, covering investigative results ranging from about 30 years ago to a relatively recent period. The field transformers treated were diverse in terms of their manufacturing year, manufacturer, structure, and voltage class. Despite the examination results of such wide-ranging transformers, there were similar changes in various characteristics due to the aging of insulating oil and paper. For example, as for the insulating oil, due to the aging for about 30 years, the characteristics of the total acid value and volume resistivity had degraded, whereas the breakdown voltage decreased only slightly. Regarding the insulating paper, mechanical characteristics such as the average polymerization degree (DP) were subject to age-related decline, whereas the breakdown voltage did not significantly decrease as with insulating oil. Therefore, based on use of about 30 years, since the life of a transformer is determined by factors such as the degradation of mechanical strength due to thermal stress, insulation degradation is considered less likely to pose a problem at an early stage. Even though it is difficult to directly measure the degree of degradation of insulating paper, this can be indirectly estimated by measuring the CO2 and CO content and furfural content in insulating oil. On the other hand, given the increasing number of transformers remaining in service over 30 years, which had been deemed the expected service life, consideration must be taken to aging degradation of the insulation performance. It emerged that, in order to identify the degree of degradation of these transformers, it was effective to manage the volume resistivity and dielectric loss tangent trends in electrical terms, the interfacial tension trend in physical terms and the total acid value trend in chemical terms. Analysis of the insulating oil components is also considered an effective means of evaluating aging degradation. [ABSTRACT FROM AUTHOR]

The lightning-impulse voltage test of electrical equipment is specified in IEC 60060-1 "Highvoltage test techniques", whereby the standard waveform has a front time and a time to halfvalue of 1.2/50 μs. These values have been unchanged from IEC Ed. 1 in 1962 through to Ed. 3 in 2010, and further study is also ongoing with 1.2/50 μs as a starting point in the currently active WG "Adaptation of TC 42 standards to UHV test requirements". The present paper initially reviewed how studies had been conducted to establish the lightning-impulse test in the U.S.A. and investigated the flow of how the study results linked to IEC Ed. 1 together with the standards in Europe. Eventually, it seems that, in terms of the flow from the U.S.A., the current numerical values of 1.2/50 μs were most probably derived from the lightning surge observation results at the Wallenpaupack-Siegfried 220-kV system of the Pennsylvania Power & Light Company in 1929 and so forth. Subsequently, major characteristics of recent lightning surge observation results were analyzed from the perspective of current actual electrical facilities and equipment. Consequently, points common to the data of 1929 in terms of the average values and the breadth of the distribution emerged in the crest value - front time and crest value - time to half-value plots. At the same time, differences due to the individual conditions of the individual measurement positions were also recognized. The detailed analysis and study of the standard waveform of the lightning-impulse test based on the analysis results are considered issues to be addressed. [ABSTRACT FROM AUTHOR]

The soundness of a power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration induced ac voltage test. The acceptance criterion for this PD test is 500 pC according to IEC standard; however, few basic data backing this criterion are available. To establish a clear criterion for this PD test, the authors initially conducted a study on the harmful PD level of materials themselves constituting the insulation of an oil-immersed transformer and a gas-filled transformer. This PD level was evaluated based on the rate of decline in the lightning impulse breakdown voltage using the insulating materials themselves and the element models simulating the insulating structure which had been exposed to a PD. Consequently, it emerged that insulations of both types of transformers were degraded if exposed to a PD of 7,000 pC to 10,000 pC. With the safety factor for this PD value taken into account considering the long-term operation and the structural difference of an actual transformer, 5,000 pC was deemed as the harmful PD level at the PD occurrence position. Subsequently, using a winding model of a transformer, PD signal propagation characteristics inside the winding were investigated through actual measurement and analysis. As a result, it was found that the PD having occurred inside the winding is measured as the signal significantly damped depending on the position of occurrence. The transmission rate was 2.2% for an oil-immersed transformer and 2.8% for a gas-filled transformer in the respective lowest cases. What should be controlled in the PD test of an actual transformer is the value of the harmful PD level at the PD occurrence position multiplied by the transmission rate at the PD detection position. Therefore, the conclusion was reached that the acceptance criterion in the test should be set to 5,000 pC ??? 2.2% = 110 pC or less for an oil-immersed transformer and 5,000 pC ??? 2.8% = 140 pC or less for a gas-filled transformer, respectively. [ABSTRACT FROM PUBLISHER]

The transient grounding impedance of a gas-insulated switchgear (GIS) substation was investigated through measurements on a commercial 300-kV underground substation. Based on the results, the macroscopic transient impedance of the grounding grid was clarified to be the inductance component and estimated at approximately 0.2 \muH. In the low-frequency region where the frequency is almost zero or so, the impedance is shown to be lower than the general concrete resistance value by one digit due to the complexity of grounding systems specific to underground substations. Hence, the simple model of the grounding grid can be expressed mainly by inductance. The detailed model was expressed by using a \pi-type equivalent circuit, where each circuit constant was calculated using the specific low resistivity of concrete. In addition, the validity of both models was confirmed by comparing the measurement results. The proposal models are applicable to the Electromagnetic Transients Program analysis on the surge phenomena within the GIS grounding system. [ABSTRACT FROM AUTHOR]

In recent years, compact gas-insulated switchgear (GIS) has been developed by rationalization of specifications. All of the GIS have been checked for passage of the AC and lightning impulse (LI) withstand test, but DC stress that occurs on an AC GIS spacer when switching devices are operated has not been sufficiently investigated. Since the electrical field strength inside a GIS tank is stronger in compact GIS, it is increasingly necessary to evaluate the withstand performance of GIS spacers under residual DC voltage. Under DC stress, electrification strengthens the electrical field strength at the creepage surface ( Et) and the withstand performance of GIS spacers depends on the maximum value of Et ( Etmax). When the volume resistivity at the surface layer of spacer is uneven, Et strengthens at the uneven spots. In this paper, Etmax under DC stress with uneven volume resistivity at the surface layer of the spacer is investigated, and an evaluation test method for withstand performance under residual DC charge is constructed. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 179(1): 21-28, 2012; Published online in Wiley Online Library (). DOI 10.1002/eej.21233 [ABSTRACT FROM AUTHOR]

Electric supply reliability is an issue of wide importance to both an information-oriented society and electric power companies. This paper focuses on lightning effects on distribution lines. Field research is now underway in the northern part of the Kanto Plain in Japan, and voltage and current waveforms in distribution lines due to lightning hits have been observed since 1996. There are now 284 datasets; these include data on 62 direct flashes to lines. This research reveals that a distribution line can be protected from direct lightning strokes. In some cases, no power follow current was confirmed after a multiple phase flashover, and a statistical analysis was conducted to investigate factors affecting the generation of power follow current. These results can serve as a valuable resource to help clarify the mechanisms underlying the production of distribution-line faults caused by lightning effects. [ABSTRACT FROM AUTHOR]

Recent lightning current observations have suggested that a front duration of 1.7 \mus is more appropriate than the conventional 1.0 \mus for the lightning protection design of ultra-high voltage (UHV) systems in Japan. This study examined overvoltages at gas-insulated subsystems (GIS) and transformers in a UHV (the rated voltage of 1100 kV) substation during backflashovers, with the front duration of lightning stroke current waveforms taken as a parameter. The dependence of overvoltages on the front duration of the lightning stroke current is significant at GIS and relatively small at the transformer. Generally speaking, the shorter the front duration is, the higher the voltage will be. The transformer capacitance, ranging from 0 to 1000 pF, equivalently increases the distance to the surge arrester, and a voltage increase was observed at the transformer terminal. However, the steepness of the intruding surge waveform is reduced and the voltage increase suppressed at 1000 pF and over. The effect of overvoltages on insulation design was also evaluated and the results thereof indicated the possibility of reducing the lighting impulse withstand voltage. [ABSTRACT FROM PUBLISHER]

Diagnosis on electrostatic charging tendency (ECT) of mineral insulating oil for power transformers has been performed. Increase in the ECT is derived from sulfonium ions, which are generated by sulfoxide compounds and hydrogen ions. Strong correlation between the ECT and the sulfoxide concentration is obtained in a series of thermal aging tests with oxygen and/or copper catalyst. The volume resistivity of oil is used as an alternative indicator for the concentration of hydrogen ion because hydrogen ions increase the electrical conductivity of oil and the sensitivity of conventional property such as total acid value is too low to evaluate the aging deterioration of closed-type transformers. Analysis of oils extracted from 98 actual transformers, whose operating years are between 3 and 46 years, are performed. High ECT values more than 200 pC/ml are detected in some oils and their volume resistivity values are less than 4 x 1014 Ωcm. Degree of the correlation between the ECT and the sulfoxide concentration is low compared to the result of the thermal aging tests. A diagnostic map with four areas classified using a pair of indicators of volume resistivity and sulfoxide concentration is useful to predict prospective increases in the ECT. Appropriate transformer operation and maintenance including air exposure avoidance of their mineral insulating oils is one of the effective solutions to prevent the ECT from increasing. [ABSTRACT FROM AUTHOR]

Additives to insulating oil and oil treatment by absorbents are investigated by thermal aging tests with the aim of suppressing flow electrification in power transformer insulation. 1, 2, 3-benzotriazol (BTA) is found to be effective for suppressing the increase of electrostatic charging tendency (ECT) in oils that contain either sulfide or sulfoxide compounds. Sulfide compounds are contained in new oil while sulfoxide compounds are generated by the oxidation of sulfide compounds. However, 2, 6-ditertiary-butyl paracresol (DBPC) is only effective for oils that contain sulfide compounds. The addition of sulfoxide compounds significantly increases the ECT of DBPC-added oil by the thermal aging tests. DBPC can suppress the conversion of sulfide to sulfoxide by blocking the generation of peroxide. However, DBPC has negligible effect on suppressing the increase in ECT by sulfonium ions, which are generated by the oxidation of sulfoxide. Negatively charged BTA ions, which are generated by the dissociation of hydrogen ions from BTA molecules, can mitigate the increase in ECT by sulfonium ions, which charge insulating oil positively. The effect of BTA is found to be lost when the concentration in oil decreases due to the adsorption on solid insulators. Oil treatment by absorbents such as clay, activated carbon and silica gel is found to remarkably decrease the ECT of deteriorated oil. However, additional thermal aging test increases the ECT to the same level as before the oil treatment. Although the additives and the oil treatment by absorbents are effective for suppressing and decreasing the ECT of insulating oils, their long-term effect is not assured. Thus periodical measurements of the ECT of insulating oils are crucial for the sake of preventive maintenance of aged transformers. [ABSTRACT FROM AUTHOR]

To identify the aging characteristics of oil-immersed transformer oil in long-term operations, it is important to survey various characteristics of insulating oil of field transformers. The insulating oil characteristics defined in various standards can be roughly classified into electrical, physical, and chemical characteristics. In a past study, the authors identified through field survey what kind of characteristics receive significant impacts as a result of insulating oil aging and evaluated the characteristics as possible candidates for indexes of aging. This paper examines the elements that influence the characteristics of transformer insulating oil due to aging. The electrical characteristics (breakdown voltages, volume resistivities, and dielectric loss tangents) of insulating oil are affected by such elements as water contents and electrostatic charges and possibly by trace components in the insulating oil. The components detected in field transformer oil are methyl acetate, 2-methylfuran, phenol, methyl formate, furan, methanol, ethanol, acetone, isopropyl alcohol, and methyl ethyl ketone. Five of these components, i.e., methanol, ethanol, acetone, isopropyl alcohol, and methyl ethyl ketone, are alcohols and ketones. These five components are generated by age-related oxidation. The five components, except for methanol, was found to have age-related increasing tendencies. Methyl acetate, 2-methylfuran, phenol, methyl formate, and furan are considered to be components mainly generated by the reason of insulating paper aging. Whereas 2-methylfuran and furan were found to have agerelated increasing tendencies, other components were found to have none. Analysis of insulating oil components is effective in diagnosing the aging status of insulating oil and the presence or absence of abnormalities. This paper summarizes the result of survey on the relationships between the concentrations of components and the ages of insulating oil based on the analysis results of the oil components. [ABSTRACT FROM AUTHOR]

Electrostatic charging mechanism in aged oil-immersed transformers is investigated. Sulfonium ions, which are generated by sulfoxide compounds and hydrogen ions, are identified as the prime compounds that increase electrostatic charging tendency (ECT) of mineral insulating oil. Sulfoxide compounds are generated by the oxidation of sulfide compounds that are contained in new oil. Hydrogen ions are considered to be supplied by organic acids that are generated due to the oxidation of hydrocarbons. Negative ions with low mass are generated by the dissociation of organic acids during the generation of sulfonium ions. Because these negative ions are hydrophilic, the oil is charged positively. Water is also generated by the deterioration of insulating oils and has the same effect. On the other hand, negative charging of oil is detected with the addition of sulfonic acids, which reflects the generation of positive ions with low mass and hydrophility. [ABSTRACT FROM AUTHOR]

Accurate predictions of lightning surge overvoltages are essential to power equipment insulation design. Recent observations of lightning strokes to uiltra-high-voltage designed transmission lines confirmed direct lightning strokes caused by shielding failure and found phenomena unexplainable by conventional shielding theories. However, there are few detailed studies of direct lightning surge overvoltages. This study assumed direct lightning stroke currents based on observational data and performs Electromagnetic Transient Program analysis of the gas-insulated switchgear (GIS) and transformer overvoltages at substations to study the basic characteristics of overvoltages due to direct lightning strokes and evaluate lightning protection design. Consequently, the maximum GIS overvoltages were found during back-flashovers, and the locations of maximum overvoltages from direct lightning strokes and back-flashovers differ. Direct lightning strokes may be more severe than back-flashovers for transformers. This paper also studied the overvoltage generation mechanism and showed the relationship of the maximum voltage to lightning stroke current and transformer capacitance. [ABSTRACT FROM AUTHOR]

The peak current amplitude and front duration of lightning stroke current waveforms have conventionally been evaluated independently, and severe waveform assumptions are usually adopted in the current of lightning insulation designs. If lightning stroke waveform conditions are optimized, lightning insulation specifications of power facilities can be further rationalized. This paper proposes a new statistical technique to better evaluate lightning stroke current waveforms. The characteristics of lightning stroke current waveforms (i.e., that peak amplitudes correlate with front durations) are taken into account in this technique. The proposed technique was validated with various statistical tests. The front duration currently used in the design of the ultra-high-voltage system in Japan is 1.0 μs, but the potentially more accurate value of 1.7 μs was found when waveforms were evaluated with the proposed technique, and overvoltage analyses of gas-insulated switchgear show that overvoltages decrease with longer front duration. [ABSTRACT FROM AUTHOR]

A widely used, effective method for diagnosing the insulation of a gas-insulated switchgear (GIS) is to detect partial discharges using signals in the ultrahigh frequency (UHF) band. To detect partial discharge signals that propagate inside a GIS, it is necessary to install a sensor inside or set on the outside of a GIS tank. For GISs that have already been constructed, it is effective to use external diagnosis using electromagnetic wave (EM-wave) signals that radiate (leak) from the insulation spacer of the GIS tank, and there is strong demand for such a diagnosis technology. Externally leaking EM-waves are often detected using an antenna, and multiple antennas supporting different frequency bands are commercially available. The accuracy of external diagnosis could be improved by comparing and evaluating the characteristics of various antennas. The insulation spacer of a GIS tank is usually installed on the flange and fastened with metal bolts. GIS tanks are made electrically continuous by these metal bolts and have the same grounding potential. EM-waves leak from the part of the solid insulator surrounded by these metal bolts and the GIS tanks. This leaking part forms a slot antenna and has a resonant frequency that depends on the spacing between the bolts in the circumferential direction of the flange. At this resonant frequency, the output of externally leaking EM-waves is higher and enables more sensitive measurement. in this study, the detection characteristics with different antenna positions and directions were compared in consideration of the number and positions of the bolts on the spacer, and the frequency characteristics of various antennas were obtained. To improve the detection sensitivity, a prototype of an improved dipole antenna was fabricated and its characteristics were evaluated. Based on the experimental results, an ideal diagnostic system based on the externally leaking EM-waves is proposed. [ABSTRACT FROM AUTHOR]

Gas-insulated switchgear (GIS) has widely been used for AC power distribution because of its high reliability and compactness. Recently, DC GIS has been developed with various investigations for dielectric breakdown characteristics of DC gas insulation. GIS insulation is composed of SF6 gas and solid spacers, and it has been recognized that the dielectric performance of DC GIS is mainly influenced by solid spacers. Under DC stress, the electric field is directed one way, the effect of electrification for charges to be accumulated in the spacer must be taken into account and also the effect exists in AC GIS because the switching operations may leave the remnant DC charge on the AC GIS spacer. This paper first describes the effective resistivity (the bulk or the surface) of the solid spacer under the DC stress from the experimental investigation, and the critical factor on the solid spacer that causes reduced dielectric performance of the GIS insulation is studied. Second, the present paper deals with the electrification on the GIS with various levels of surface roughness of the epoxy insulator and metallic electrode. Finally, the DC insulation characteristics of GIS insulator are investigated based on the experimental results. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(4): 6–13, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20788 [ABSTRACT FROM AUTHOR]

It is important to understand the interruption phenomena for contact materials in order to adapt them for use in high voltage. Accordingly, we have carried out measurements of the arc energy and anode surface temperature and observed the vacuum arc in a vacuum chamber. We have used three kinds of contact materials in this investigation, namely, Cu, CuCr, and AgWC. In addition, it is effective to develop a new contact material as the means of application to high voltage of vacuum circuit breaker. Therefore, we made a prototype of new contacts, based on the tests results, for adaptation to high-voltage vacuum circuit breakers. In addition, we carried out interruption tests using these contact materials. [ABSTRACT FROM AUTHOR]

In lightning impulse withstand voltage tests for substation equipment, waveforms regulated in the standard are sometimes difficult to generate on large equipment of the UHV class due to the level of capacitance of the test equipment "Ct" and the level of the residual inductance of the test circuit including the power supply "Ls". In the present study, the relationships between the capacitance of the test equipment, residual inductance of the circuit, the front time "Tf" of generated waveforms, and the overshoot rate "β" were obtained using a simple equivalent circuit by assigning parameters systematically. Consequently, it has been found that, while Ct and Ls are small, there is a series resistance "Rs" range in which both the Tf and β conditions are met and that, as Ct and Ls grow larger, there is an Rs range in which only one of the conditions is met but not both the conditions, and in the end even the Tf condition alone is not achieved. An increase either in β or Tf increases the testable Ct upper limit or permissible Ls limit value, and an increase in Tf is found to be more effective. [ABSTRACT FROM AUTHOR]

In lightning impulse withstand voltage tests for substation equipment, waveforms regulated in the standard are sometimes difficult to generate on large equipment of the UHV class due to the level of capacitance of the test equipment "Ct" and the level of the residual inductance of the test circuit including the power supply "Ls". In the present study, the relationships between the capacitance of the test equipment, residual inductance of the circuit, the front time "Tf" of generated waveforms, and the overshoot rate "β" were obtained using a simple equivalent circuit by assigning parameters systematically. Consequently, it has been found that, while Ct and Ls are small, there is a series resistance "Rs" range in which both the Tf and β conditions are met and that, as Ct and Ls grow larger, there is an Rs range in which only one of the conditions is met but not both the conditions, and in the end even the Tf condition alone is not achieved. An increase either in β or Tf increases the testable Ct upper limit or permissible Ls limit value, and an increase in Tf is found to be more effective. [ABSTRACT FROM AUTHOR]

Until recently, SF6 gas has widely been used as the best insulating medium in substation equipment. However, SF6 gas was specified to be a greenhouse gas at COP3 in 1997 because of its high global warming potential (GWP), and alternative insulation gases to SF6 have been sought for a long time. Alternatives using the natural gases are considered to be suitable, but none of them show better properties for insulation as well as good environmental compatibility. Therefore, it is necessary to rationalize the equipment insulation level and reduce the test voltage of electric power apparatus to as low a level as possible. The actual lightning surge waveform (so-called nonstandard lightning impulse waveform) occurring in the actual field is different from the standard lightning impulse waveform (1.2/50 μs). There are many cases in which the actual lightning surge waveform has a steep rise and large decay of overvoltage, and the insulation requirements are not as severe as those for the standard lightning impulse waveform. In this paper, we focused our research on N2 gas as an SF6 substitute and investigated the insulation characteristics of N2 gas for a single-frequency oscillatory waveform with various frequencies from 2.7 to 20.0 MHz and damping ratios. Based on the experimental results, it might be possible to reduce the test voltage of N2 gas insulation by evaluating the crest value of the actual lightning surge waveform that has been converted into an equivalent standard lightning impulse waveform. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(2): 10–20, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20754 [ABSTRACT FROM AUTHOR]

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, TE11, and TE21 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 TE11 and TE21 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 SF6 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. [ABSTRACT FROM AUTHOR]

Abstract-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 lines since 1996, obtaining 284 datasets, including 62 for direct hits to the lines. 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. [ABSTRACT FROM AUTHOR]

Probability distribution of surge discharging current of arresters provided a basis for conventional theory of grounding systems. In order to rationalize the grounding systems, it is necessary to grasp the statistical data of lightning surges on distribution lines caused by direct lightning strokes and indirect lightning strokes. Lightning phenomena on TEPCO's distribution lines had been continuously observed for the rationalization of lightning protection design of distribution lines. The observation had been carried out with still cameras and monitoring sensors of lightning surges. This makes it possible to discover new interesting facts that can be useful for rationalization of lightning protection design of distribution lines. Cumulative frequency distribution of conventional data is close to that of ZnO discharging current in the case of direct strokes and indirect strokes through TEPCO's observation. Moreover, to verify the cumulative current distribution in concrete poles, the authors have compared the cumulative distribution of current through ground lines with that of current through ground lines and concrete poles. The results show that the distribution of current through ground lines and concrete poles is larger than that of current through only ground lines for high currents exceeding 1 kA. This fact suggests that lightning surge current flows not only in ground lines but also in concrete poles. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(2): 36–44, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20468 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In order to rationalize the lightning protection design of distribution lines, lightning phenomena on TEPCO's distribution lines were continuously observed. The observations were carried out with still cameras and lightning surge monitoring sensors. The results elucidate the lightning performance of distribution lines. For example, direct lightning strokes are the major cause of the present distribution line faults. This fact suggests that it is important to investigate observation data on direct lightning strokes. The observation data were analyzed by means of a direct lightning analysis model. The calculated waveforms were compared with the measured waveforms on distribution lines. The probability distributions of the surge currents and voltages were calculated by the Monte Carlo method. These results provide new data on lightning performance. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 91(10): 28– 37, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10164 [ABSTRACT FROM AUTHOR]

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 (EM!) 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 (PS), 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. [ABSTRACT FROM AUTHOR]

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 SF6 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 SF6 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. [ABSTRACT FROM AUTHOR]

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 (00) 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 00 direction, the distribution of the EM-wave strength varied according to through the L-shape structure. Also in the case of a 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. [ABSTRACT FROM AUTHOR]

To tower 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. [ABSTRACT FROM AUTHOR]

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<1, the former gives lower values than the latter. Then, appropriate reliability evaluation methods are investigated for various insulation media and insulation structures of substation equipment from the viewpoint of inception and development mechanisms of dielectric breakdown and partial discharge. According to the result of engineering evaluation of the presently available data, the independence method may be appropriate for both gas insulated switchgear and oil-immersed transformers. [ABSTRACT FROM AUTHOR]

Currently, environmental problems such as global warming are important issues, and SF6 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 SF6 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 CO2 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 CO2 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 EMTF through the simulation of the line part by the multi-phase distributed constant and the simulation of the insulated joint by the capacitance. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

There is not sufficient data on the breakdown voltage-time characteristics for gas insulated switchgear (GIS) at power-frequency voltages, especially in the long-term region, for complete insulation specifications to be developed. This paper gives voltage- time breakdown characteristics for a 72-kV GIS at power-frequency voltages, both for clean conditions and with metallic particles added, from times of the order of a second to times of the order of 1000 hours. The Weibull parameters were calculated to be n = 95 and a = 0.32 to 0.47 in the clean case; and in the metallic-particle case n = 31 and a = 0.92 to 0.98 for a short-term region and n = 66 and a = 0.40 to 0.47 for a long-term region, with the boundary between these two regions located at 10 minutes. Based on the statistics for the metallic-particle case, the reliability of the power-frequency withstand voltage tests were then estimated using as examples the GIS in a 500-kV system (an effectively grounded system) and a 66-kV system (a non-effectively grounded system). The paper also studied the effect of the type of grounding and the influence of the presence of metallic particles on the test voltage level. [ABSTRACT FROM AUTHOR]

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 SF6 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%. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The electrogeometric model (EGM) proposed by Armstrong and Whitehead has been used to calculate the lightning shielding features of transmission lines. While the striking distance equation in the EGM uses the 50% flashover voltage characteristics of negative switching impulses for rod-rod gaps, those characteristics for long air gaps have not been sufficiently examined. The present study has therefore focused on clarifying the 50% flashover characteristics of negative switching impulses for long air gaps. Consequently, saturating characteristics have been obtained for rod-plane gaps compared with almost linear characteristics for rod-rod and rod-conductor gaps. Furthermore, new knowledge was obtained indicating that, for air gaps of around 4 to 5 m, the 50% flashover voltage may possibly be lower for rod-plane gaps than for rod-rod and rod-conductor gaps. [ABSTRACT FROM AUTHOR]