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

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

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

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

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