Surface Discharge Characteristics of Epoxy Resin in SF6 Gas under Switching and Lightning Impulse Voltages

UHV DC wall bushing is the core equipment connecting the valve hall and DC field in converter station and failure occurs to it frequently under impulse voltages. Surface flashover experiment under impulse voltages was conducted on epoxy resin, which has been widely used as insulation material in 1100kV wall bushing. Surface discharge voltages of epoxy resin under the standard lightning and switching impulse voltages at various electrode gap distances and gas pressures were measured together with the breakdown time delay by Up and Down Method. The difference of U-T curves between lightning and switching impulse voltages were compared on the base of statistical time delay and discharge development time delay calculation. It shows that when the gap is short, statistical time delay takes the predominant role, but when the distance is longer, discharge development delay takes the main role in the breakdown time, which is influenced by surface charge accumulation and decay. Therefore, the breakdown time range of U-T characteristic is dependent on the gap distance and electric field distribution, which provides a reference for the insulation design.