Evaluation of breakdown characteristics of CO2 gas for non-standard lightning impulse waveforms - breakdown characteristics under double-frequency oscillation waveforms and single-frequency oscillation waveforms in the presence of bias voltage

Abstract

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. Accordingly, in previous studies, the insulation characteristics for single-frequency oscillation waveforms were experimentally obtained in the quasi-uniform electric field, which is the basis when performing the insulation design of GIS, and its evaluation method was examined. In this paper, to further elaborate the evaluation method established thus, the insulation characteristics in relation to complex oscillatory waveforms generated in actual substations, e.g. double-frequency oscillation waveforms and single-frequency oscillation waveforms on which a dc component (bias voltage) that remains due to the superimposed operation of a switching device, were obtained. Consequently, the results obtained indicated that negative polarity waveforms were more severe in terms of electrical insulation under the double frequency oscillation waveforms as well as under the single-frequency oscillation waveforms. Therefore, when considering a dielectric strength, it is reasonable to conduct experiments using negative-polarity waveforms for the complex oscillatory waveforms, such as the double frequency oscillation waveforms. Furthermore, a study was performed on a method of evaluating insulation characteristics for non-standard lightning impulse waveforms of the CO2 gas gap via a comprehensive approach using the results including the previous ones. As a result, as in the case of studies conducted previously on SF6 gas, the insulation characteristics, including complex oscillatory waveforms generated in actual power systems, could be expressed by a single characteristic line by using the parameter of duration.