Simulation Research on the Influence of Metal Granule on Vacuum Flashover in Pre-Breakdown Stage

Metal granule can be commonly found on insulator surface for various vacuum apparatus, producing significant impact on the durability and performance of high voltage devices. A particle-in-cell simulation model is constructed to study its influences on vacuum flashover, combined with experimental verification. A series of sizes and positions of metal granule are tested showing two competitive leverages of metal granule namely field distortion near cathode triple junction for bigger granule near cathode, and a negative charge concentration due to specific secondary electron emission characteristic, which blocks multipactor expansion and reduces surface charge accumulation for granule with smaller size positioned away from cathode. The net effect on flashover voltage is twofold and has to be analyzed considering both factors. Possible degradation of flashover threshold is proposed for granule with typical size formed by transient vacuum arc. In addition, if the negative effect with respect to flashover strength can be avoided with adequacy, a possible design regarding insulator structure can therefore be derived for a stronger electric strength on surface.