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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]

We have investigated the performance of silicone oil as an insulation medium with the aim of developing environmentally-friendly, flame-retardant electric power equipment. In this paper, the insulation characteristics of silicone oil and ester oil, which is the alternative insulating oil candidate, have been investigated and compared. The insulation characteristics examined in this paper are the streaming electrification characteristics including electrification phenomena generated when an insulating oil streams, dielectric breakdown characteristics with increasing temperature and the surface breakdown characteristic when the insulator is placed between electrodes. The range of temperature investigated was 30 to 130 °C. Both kraft and aramid paper were used for the insulator. In the comparison between silicone and ester oil, it was found that the insulation performance of the ester oil was better than that of the silicone oil. We have considered possible mechanisms for these results. [ABSTRACT FROM AUTHOR]

This paper investigates simultaneous measurements of partial discharge (PD) current and the corresponding light emission phenomena of C3F8/CO2 gas mixture under a non-uniform field with ac high voltage application in order to discuss the PD properties and breakdown mechanism of new gaseous dielectrics as SF6 substitutes. Perfluoro carbon (PFC) gas mixture is a possible candidate as SF6 substitute because it almost fulfills the environmental aspects, dielectric properties, boiling point, and other requirements of SF6 substitute. The authors simultaneously measured PD phenomena such as time-sequential PD current pulses and corresponding light emission images before breakdown during the positive half cycle of the applied ac voltage where most breakdowns occurred. Consequently, it was found that there were three kinds of current pulse patterns, which depended on the gas pressure and the applied voltage amplitude. It was also confirmed that breakdown occurred around the voltage peak, i.e. 90 degrees. of the applied voltage, irrespective of the PD pattern. Furthermore, the simultaneous measurements enabled the authors to classify the discharge area in the N-shaped characteristic of the breakdown voltage based on discrimination of discharge types between streamer and leader discharges. Namely, the location where leader discharge is generated by the transition from streamer discharge in the N-shaped characteristic was indicated. These experimental results and the simultaneous measurement technique will help clarify the breakdown mechanism for new gases or gas mixtures as SF6 substitutes. The obtained results will also contribute to insulation diagnosis for next-generation gas-insulated power apparatus with the newly developed gaseous dielectrics. [ABSTRACT FROM AUTHOR]

The authors previously pointed out that the results of insulation tests based on the one- minute step-up method deviate from the "true values". In the present study, these deviations are evaluated by numerical simulations. These deviations were found to exist not only under limited conditions in numerical simulations but to exist fundamentally on the one-minute step-up test method. This should be taken into consideration when making actual insulation designs. [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]

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 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. 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 non- uniform 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. [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]

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. This paper describes obtaining through experiments the insulation characteristics for single-frequency oscillation waveforms as a representative waveform when there is a non-uniform electric field represented by metallic particles in the equipment while changing the applied voltage polarities, frequencies, and damping factors. As a result, it was found that the dielectric breakdown voltages are lower in the positive polarity and it was confirmed appropriate to conduct experiments mainly in positive-polarity in order to consider dielectric strengths. Furthermore, changing the frequencies and damping factors under positive-polarity waveforms did not cause major changes in the feature of the dielectric breakdown voltages, which remained constantly higher (by 1.41 to 2.58 times) than those under standard lightning impulse waveforms. [ABSTRACT FROM AUTHOR]

Because of the excellent high insulation performance, vacuum is expected to be one of the alternatives of SF6 gas. In order to reduce the use of SF6 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. [ABSTRACT FROM AUTHOR]

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 "σ-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%. [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]

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. [ABSTRACT FROM AUTHOR]

Gas insulated switchgears (GISs) with SF6 gas with superior insulation and interruption performances used as an insulating medium have realized significant downsizing of substation facilities. GISs have been in use in substation facilities for more than 30 years and are expected to be used for an even longer period of time in the future. To realize effective operation and maintenance of highly aged apparatuses, it is necessary to establish insulation diagnosis technology that can be used to detect and accurately assess a discharge phenomenon occurring in a GIS. An effective diagnosis method for the prevention of a facility contingency is to detect a partial discharge (PD) that occurs as a precursory phenomenon of dielectric breakdown. Whereas many of the diagnosis technologies for a PD are based on electrical measurements, the focus in this study was placed on the insulation diagnosis for decomposed gases generated by the energy of a discharge. Chemical measurements for detecting decomposed gases are less influenced by noise and more reliable than electrical measurements. Furthermore, an abnormality can be detected even if a PD does not occur at the time of measurement because decomposed gases build up. This method can also be applied to a fault location. This paper describes creating the prototypes of decomposed gas sensors based on solid electrolytes to detect fluoride gases. The sensors with different output types, i.e., "voltage-output type" and "current-output type", were examined. The "voltage-output type" is suited for diagnosis in a wide range of concentrations whereas the "current- output type" has a good detection sensitivity in a low-concentration range. Furthermore, the "hybrid" sensor, which features a combination of the "voltage-output type" and "current-output type", was prototyped and evaluated, resulting in a finding of a good development possibility for a sensor that has both the advantages of these two types without impairment to either of them. [ABSTRACT FROM AUTHOR]

SF6 gas has excellent dielectric strength and interrupting capability and is used in various applications such as gas insulated switchgear (GIS) in substations. However, since SF6 has a high global warming potential (GWP), it is imperative to reduce its use and develop recovery technology for its reuse. This paper examined the potential of alternative insulating gases for GIS, determining experimentally the ac and lightning impulse breakdown characteristics of CO2 and N2 gases, which have little environmental impact, in a quasi-uniform electric field. The effect of electrode surface roughness is greater in N2 than in CO2. The impulse ratio in a quasi-uniform electric field was smaller in N2 than in CO2 at low pressure, but this reversed when the pressure reached 1.2 MPa. The dielectric strength of an electrode with a dielectric coating was approximately equal to that of a specular-finish bare electrode in either CO2 or N2 when an ac voltage was applied. When lightning impulse voltage was applied, a significant improvement of dielectric strength was observed if a dielectric coating was applied to the electrode considered to be the initial electron source. [ABSTRACT FROM AUTHOR]

For the insulation diagnosis of gas-insulated switchgears, the UHF method, which detects electromagnetic wave (EM-wave) in the UHF band (300 MHz to several GHz) with little noise influence, has rapidly developed in recent years. The study in this paper has proposed the evaluation of the location of adhering metallic particles based on the theory of EM-wave propagation by only using the time-waveform information of the EM-wave emitted due to partial discharge, and also analyzed and studied the validity using the FD-TD method. As a result, the duration of the time-waveform differed with respect to the location of the adhering metallic particle, and this difference was caused by the difference in the relative electric field intensity at the tip of the metallic particle among EM-wave modes and the difference was more when the tank diameter was larger. Based on this result, the method for evaluating the location of metallic particle according the time-waveform was studied. In addition, the characteristics of the leakage EM-wave at the spacer aperture were analyzed, and based on the result obtained, two types of antennas, patch and waveguide slot antennas, achieving compactness and a high sensitivity were proposed. [ABSTRACT FROM AUTHOR]

In order to attain reduction in insulation test voltages, voltage-time (V-t) characteristics for ac voltage and voltage-number (V-N) characteristics for lightning impulse voltage were experimentally investigated on the turn-to-turn insulation model, coil-to-coil insulation model and barrier-oil-duct insulation model of core- and shell- type transformers under non-standard of field-use conditions. The conditions used for experiments on insulation characteristics in view of field-use consisted of "clean oil" and "hot oil" as the conditions for insulating oil, "trapezoidal waves", "alternate applications of positive and negative voltages", and "steep front waves" as the conditions for applied voltages, and "low dielectric constant pressboards" as an insulating object in experiments. Although the measured V-t and V-N characteristics varied depending on the conditions, the total evaluation in terms of insulation coordination showed that results under standard conditions were satisfactory in almost all the cases and some cautionary points in a few cases were recognized. [ABSTRACT FROM AUTHOR]

Numerical simulation of electromagnetic fields was performed to investigate the transmission and reflection characteristics of electromagnetic waves excited by partial discharge through spacers, L-shaped sections of a GIS bus structure. The resonance frequencies of a cavity between spacers agreed roughly with the experiment. The modes of electromagnetic wave are shown to transform to different modes, e.g., from TE11 to TE21 mode, after transmission and reflection through L-shaped sections of a GIS. The frequency intervals of the resonance in tank's longitudinal direction for a L-shaped section are different between horizontal and vertical excitation of partial discharge source. The two characteristics, mode transformation and the dependence of resonance frequency on excitation direction, were confirmed experimentally. [ABSTRACT FROM AUTHOR]

This paper describes the propagation characteristics and mechanisms of partial discharge (PD) leading to breakdown (BD) in electronegative gases such as SF6/N2 and C2F6/N2 gas mixtures for ac voltage application. An established PD measuring system of highly time-resolved PD current and light intensity waveforms, streak and light emission images enabled us to discuss the physical mechanisms of PD generation and propagation in terms of space charge behavior. Experimental results revealed that the time-resolved PD generation was divided into different groups composed of the first large PD and the subsequent small PDs; the former was identified as the leader discharge, and the latter corresponded to the streamer discharges. The successive generation of the streamer and leader discharges was interpreted by the change of critical volume size. The time-resolved PD generation characteristics leading to BD were also measured and discussed. The leader discharge would be obliged to take a detour along the periphery of the positive space charge area in the vicinity of the needle electrode, and finally result in BD in electronegative gases. [ABSTRACT FROM AUTHOR]

Insulating materials exist in a variety of different forms, including gas, liquid, solid, and vacuum, and different forms of insulating materials have significantly different dielectric strengths and insulating properties. Power transmission/substation facilities for UHV AC and 500 kV DC power transmission and 500 kV underground power transmission facilities have been designed and constructed by taking advantage of the insulating properties of various insulating materials and structures so that they conform to the demanded insulation specifications. Conventionally, insulating material technology has been developed by making a close study of apparatuses and insulating materials individually. In this paper, as a new attempt, the insulating properties of various insulating materials/apparatuses are cross-sectionally compared and examined from an entirely new viewpoint through the assessment of commonalities and differences between them. A listing of lightning impulse and AC dielectric strength data is prepared and systematically positioned from the standpoint of material and structure, and compared and evaluated in a unified way from the viewpoint of typical indices of insulating properties including the ‘n’ value of the V-t characteristic, Weibull parameters, and impulse ratio. Further, an insulation assessment of apparatuses is systematically made with consideration given to their sizes and structures as well as the insulating materials themselves. As a result, it was found, for example, that the design field strengths of power capacitors were high and those of generators were not, and there was a noticeable difference in design field strength between SF6 gas insulated apparatuses and oil-immersed transformers. It is expected that the analysis conducted from an entirely new angle may provide a new approach to the development of apparatus insulation technology and to the rationalization of insulation design. [ABSTRACT FROM AUTHOR]

This paper deals with consideration of particle motion control and its trapping in the deactivation of foreign contaminating moving particle in simulated GIS. The particle motion behavior in non-uniform field gap without/with different shaped spacers was investigated experimentally under dc and 60 Hz ac voltages. The obtained results were confirmed experimentally as well as by calculating the electrostatic force acting on the particle in the non-uniform field gap and are reported elsewhere. Whereas, in this paper, the observed particle motion behavior around simple shaped spacer is simulated by solving the particle motion equations analytically in the presence of electrical gradient and image forces. The simulation agreed fairly well with the experimental results. Furthermore, in the non-uniform electric field, the effect of dielectric coating on the grounded electrode on the particle motion and particle deactivation is also briefly discussed. On the basis of the obtained results of particle motion behavior without/with, different shaped spacers and the effect of dielectric coating on the ground electrode, different methods of particle deactivation are suggested as well as confirmed experimentally. [ABSTRACT FROM AUTHOR]

High lightning overvoltages do not appear in underground substations connected to transmission cables. Consequently, it is very important to thoroughly investigate switching overvoltages and to achieve rational insulation coordination for apparatus installed in such underground substations. This paper discusses the occurrence of steep fronts at transient recovery voltages (TRV) appearing at circuit breakers when the inrush currents of transformers are interrupted. Caused by a steep front at the TRV, reignitions occur at circuit breakers, resulting in the generation of high overvoltages with high frequencies. The overvoltages are among the highest switching overvoltages appearing at the terminal of a transformer. The authors clarified the mechanism of the generation of steep fronts at TRV by means of EMTP analysis, as well as by carrying out tests in a high-power laboratory. © 2001 Scripta Technica, Electr Eng Jpn, 135(1): 24–32, 2001 [ABSTRACT FROM AUTHOR]

To discuss the UHV GIS test voltage, the deterioration characteristics of the GIS insulating spacer model were studied by repeating lightning or switching impulse voltage. The deterioration factor indicating the decrease of lightning or switching impulse breakdown voltage applied up to 1000 times was found to be 35.7/45.5. The relationship between the partial discharge inception voltage and the breakdown voltage after 1000 impulse applications was also studied. It was found that the partial discharge inception voltage increases despite a decrease in breakdown voltage due to the impulse voltag. In addition, to elucidate the deterioration phenomenon, a study was made of the breakdown characteristics of models with various boundary conditions between the electrode and epoxy resin, as well as the relationship between the boundary conditions. Microdischarge was also studied. The results of the investigation suggest that the deterioration by repeating voltage is caused by microdischarge due to electric field intensification on microprotrusions of the electrode surface. [ABSTRACT FROM AUTHOR]

The article discusses flashover properties and calculated values of binary mixtures of mixed gas. Four gases SF6, N2, O2 and Ar were examined. The calculated values were then compared with the experimental values and the synergism for each combination. SF6, N2 are important insulation gases. Ar is useful for illuminating tubes and mixing SF6 with this is being considered for the purpose of raising dielectric strength. Atmospheric O2 is an insulating medium and mixes frequently as an impurity in SF6 in practical cases.

The paper reports a new transversal study of dielectric coating effects on high voltage (HV) electrodes in SF6 and eco-friendly gases like CO2, dry air and N2 for real-size gas insulated switchgear (GIS). From a practical viewpoint, the breakdown electric field (BDE) data of the following test conditions are collected; 1) large effective electrode area over about 5 x 104 mm2, 2) actual GIS cylindrical configuration of utilization factor of around 0.6, 3) high gas pressures up to 1.9 MPa for actual GIS. The BDEs of dielectric coated electrodes relative to those of bare electrodes (normalized BDEs) are systematically evaluated as a function of dielectric coating thickness and gas pressure. As a result, the normalized BDEs in percentage terms are 105-150% (average value of around 110%), which does not depend on the gases. Remarkable performances of the normalized BDEs of 120-150%, which correspond to nearly 90% of the theoretical BDEs of the gases, are observed in SF6 and CO2 for "thick casting epoxy coating", "thin coating of comparable dielectric performance to casting epoxy" and "hard anodic-oxide coating & PTFE-immersion" that have fewer defects inside. The normalized BDEs in N2, however, show poorer dielectric performances than the other gases especially in the high pressure region above 1.0 MPa. The dielectric coating effects are more pronounced as increasing the thickness for all the gases. The pressure dependences of the dielectric coating effects are not clearly recognized for all the gases and all the coating conditions except for the "thick casting epoxy coating". The coating effect in N2 decreases slightly as raising gas pressure. The application examples and application limits of the dielectric coatings to GIS are also presented. These new cross-sectional studies of dielectric coated electrodes are an effective approach to get new findings to realize a future eco-friendly compact GIS. [ABSTRACT FROM PUBLISHER]

SF6 gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative is required from an environmental perspective. As one of its potential alternatives, the authors are focusing on N2 gas, which has relatively good insulation characteristics among natural gases and a low environmental impact. However, to use this N2 gas for actual GIS, the insulation characteristics for actual overvoltage waveforms generated in the field (non-standard lightning impulse waveforms; non-standard-LIWs) must be obtained. The present study, with single-frequency oscillation waveforms as a representative basis among non-standard LIWs, experimentally obtained the insulation characteristics of an N2 gas gap by changing the frequency and the damping factor. Consequently, the breakdown voltage (BDV) was lower for the negative polarity under a quasi-uniform electric field, hence the conclusion that it was reasonable to conduct experiments using negative polarity waveforms in order to discuss the dielectric strength. In addition, even if the frequency and damping factor changed, the BDV remained relatively constant, and that is consistently higher than that for the standard lightning impulse waveforms (standard-LIWs) at a level of 1.14 to 1.24 times. As a result, it was also confirmed for GIS using N2 gas that the insulation specification could be rationalized by about 20% by converting lightning impulse waveforms from non-standard to equivalent standard-LIWs of 1.2/50 μs. Furthermore, insulation characteristics were obtained under the pattern in which a disconnector switching surges are superposed on residual dc elements generated through the operation of disconnectors (bias voltages). Consequently, the bias voltage had only a minor influence on the insulation characteristics and it emerged that insulation characteristics were likely to be evaluated using a single polarity waveform. [ABSTRACT FROM AUTHOR]

SF6 gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative means from the environmental perspective. The authors are focusing on CO2 gas, which has a lower global warming potential (GWP), as one of its potential alternatives. In order to use this CO2 gas for actual GIS, the insulation characteristics for actual overvoltage waveforms generated in the field (called non-standard lightning impulse waveforms) must be obtained. For this purpose, the preceding study experimentally obtained and evaluated the insulation characteristics for relatively low frequency oscillation waveforms in disconnector switching surges generated in the actual field. In this paper, the insulation characteristics of the CO2 gas gap for higher frequency oscillation waveforms were experimentally obtained while changing the frequency and damping rate. Consequently, in the high frequency area, if the frequency was raised or the damping rate increased, the dielectric breakdown voltage tended to increase, consistently remaining higher than that for standard lightning impulse waveforms at a level of 1.08 to 1.36 times. Accordingly, it was found that the insulation specification could be rationalized by approximately 10% by converting non-standard lightning impulse waveforms to equivalent standard lightning impulse waveforms. In addition, the influence of gas pressure and gap length was examined and it emerged that the results obtained under basic experimental conditions were likely to be applicable to the conditions close to actual GIS conditions. [ABSTRACT FROM AUTHOR]

SF6 gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, hence the search for an effective alternative means from the environmental perspective. One of the alternative candidates is CO2 gas, which has a lower global warming potential. In order to use this CO2 gas for actual GIS, the insulation specification must be rationalized by lowering the lightning impulse withstand voltage because the dielectric strength of CO2 gas is lower than that of SF6 gas. To lower the lightning impulse withstand voltage of 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 (non-standard-LIWs) that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform (standard-LIW) quantitatively. In this paper, with single-frequency oscillations as typical examples of non-standard-LIWs, the insulation characteristics for the CO2 gas gap were experimentally obtained while changing the frequency and damping rate. Consequently, even if the frequency and damping rate were changed, the dielectric breakdown voltage varied little, consistently remaining higher than that with standard-LIW at a level of 1.08 to 1.17 times. Accordingly, it was found that, for GIS using CO2 gas, the insulation specification could be rationalized by approximately 10% by converting non-standard-LIWs to equivalent standard-LIWs. In addition, the influence of gas pressure and gap length was examined and it emerged that the results obtained under basic experimental conditions were likely to be applicable to the conditions close to actual GIS conditions. [ABSTRACT FROM AUTHOR]