Platform Phenomenon of Insulation Characteristics of SF6 under AC Superimposed Impulse Voltage

Lightning impulse withstand test and power-frequency voltage withstand test are adopted to detect inner insulation defects of Gas insulated switchgears (GIS). During operation, GIS withstood power frequency voltage and impulse voltage caused by switch operation and lightning intrusion, resulting in unexpected insulation accidents under the effect of superposition. For this reason, this paper studied the influence rules of SF6 gas gap insulation characteristics under AC superimposed impulse voltage through experiments. Results show that: Under slightly non-uniform electric field, the amplitude of AC component hardly affects the insulation characteristics of the SF6 gas gap under AC superimposed impulse voltage. Under severe non-uniform electric field, there are two stages depending on whether the amplitude of the AC component is greater than corona onset voltage. When the amplitude of the AC voltage component is less than the corona onset voltage, the AC component has almost no effect on the gas gap breakdown voltage and a plateau phenomenon occurs in the meanwhile. The disappearance of the platform period is related to the change of the space charge distribution formed by AC corona, and when the amplitude of AC component is high, the space charge becomes a dominant factor which affects the insulation characteristics of SF6 gas gap.