Al2O3-Epoxy Flashover Characteristics under Positive and Negative Nanosecond Impulse in SF6

Surface flashover of dielectric materials often causes extensive damage to pulse power devices. Nowadays, this insulation issue is exacerbated by the demands for ever-higher operating voltages. To solve this issue, the flashover voltage and the streamer development characteristics were investigated under positive and negative nanosecond impulse, respectively, in pressured SF6 gas. First, the surface flashover strength was obtained and the Weibull distribution is introduced to describe the statistical characteristics. The results showed that the negative flashover voltage was much higher than the positive one for Al2O3-epoxy. Within the range of 0.1–0.3 MPa, the negative voltage increased with the gas pressure, while the positive one performed independent with pressure. Then, to investigate the streamer evolving process, a high-speed camera was used to get the discharge picture and the instant velocity of streamers at different conditions. Surface morphology change after the flashover was also analyzed. At last, the possible mechanisms for polarity and pressure effects were given.