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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]

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]

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]

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]

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]

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]

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]

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]