Degradation of cross linked polystyrene by repetitive impulse surface flashovers in vacuum

In order to understand aging mechanism of solid insulating materialscaused by surface flashover, this paper presents the electrical degradation characteristics of cross linked polystyrene (XLPS) by repetitive impulse surface flashovers in vacuum. The surface of samples underwent flashover repeatedly under different impulse voltage situations. Each flashover voltage and current was recorded, and the flashover energy was calculated to present flashover properties under each condition. Experimental results indicate that there are different degradation phenomena of XLPS after undergoing those impulse flashovers. The flashovers with small current have positive effect on surface flashover property of insulating material. However, the large currents when flashovers occurred play a destruction role in surface properties of XLPS. It is said that finite numbers of repetitive impulse flashovers under small flashover currents removed the contaminants, water and gas on the sample surface. In addition, it represents a "conditioning" effect and stable situation of flashover properties under these situations due to removal of emission sites, micro protuberance of electrodes and sample surface, especially at the cathode triple junction. However, repetitive flashovers under high flashover current (≫1 kA) led to surface degradation. This phenomena attributes to the high flashover energy changed the sample surface state, destroyed the sample surface and formed apparent carbonized channels and regions on sample surface. So flashover becomes unstable in this situation. Through summarizing and analyzing the experimental results, the flashover energy was proposed as a main factor to characterize the phenomenon of degradation by repetitive impulse flashovers in vacuum.