Evaluation of breakdown characteristics of CO2 gas for non-standard lightning impulse waveforms - breakdown characteristics in the presence of bias voltages under non-uniform electric field

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. The authors focus particularly on CO₂ gas, which has a low global warming potential, as one of these potential alternatives. In order to apply this CO₂ gas to actual equipment, the insulation characteristics for overvoltage waveforms generated in an actual field (called non-standard lightning impulse waveforms) must be obtained. The present study obtained and evaluated the insulation characteristics where a disconnector switching surge was superimposed over the residual dc component (bias voltage) caused by operating a switchgear under a non-uniform electric field, typically represented by metallic particles. Consequently, it was concluded that the breakdown voltage was lower for positive polarity under a non-uniform electric field in the presence of a bias voltage and that it was appropriate to conduct an experiment using a positive polarity waveform in the study of dielectric strength. Furthermore, it emerged that the breakdown voltage for a positive polarity waveform was 1.05 to 1.64 times higher than that for the standard lightning impulse waveform, even if the frequency and damping rate were changed.