Evaluation of breakdown characteristics of CO2 gas for non-standard lightning impulse waveforms - breakdown characteristics for double-frequency oscillation waveforms under non-uniform electric field

Since SF6 gas, an insulation medium used for gas insulated switchgear (GIS), has a high global warming potential, an effective alternative is sought from an environmental perspective. The authors are focusing on CO₂ gas as a potential alternative, given its relatively good insulation characteristics among gases with a low environmental impact (natural gases). The present study obtained and evaluated the insulation characteristics for complex waveforms, such as double-frequency oscillation waveforms, generated in an actual substation under a non-uniform electric field typically represented by metallic particle. Consequently, it was emerged that the breakdown voltage for a positive polarity waveform was 1.1 to 1.6 times higher than that for the standard lightning impulse waveform; even if parameters such as the frequency and damping rate were changed within the range assumed in the field. Furthermore, a study was conducted on a means of evaluating the insulation characteristics of the CO₂ gas gap for non-standard lightning impulse waveforms via an approach involving comprehensive handling of various waveforms, as was applied to the experimental results accumulated to date. Consequently, using a parameter of so-called duration, the breakdown characteristics for various waveforms, including complex oscillation waveforms generated in actual systems, could be expressed using a single characteristic line.