Characteristics of long gap surface flashover channel in vacuum under nanosecond quasi-square pulses

High-voltage vacuum insulator failure is generally due to surface flashover rather than insulator bulk breakdown. To study the characteristic of surface flashover channel, a diagnosis system to obtain the images of flashover channel is fabricated using a high speed frame camera, and results are presented by an investigation into the flashover of poly-methyl methacrylate insulator with gap spacing of 170 mm in vacuum under 180 ns quasi-square pulses. It is obtained that after formation of the flashover channel, the impedance of the flashover channel is inductive and the waveform of the measured voltage is determined by the inductance of the flashover channel directly.