Breakdown Phenomena Triggered by Microparticle in Vacuum Gap

Vacuum circuit breakers are widely used in the medium voltage class. In vacuum circuit breakers, late breakdowns sometimes occur after the interruption of current, and these late breakdowns are thought to be triggered by microparticles. In this paper, the motion of the microparticles and the breakdown triggered by the microparticles are observed under DC voltage. It turns out that the motion of the microparticles can be grouped into three patterns: attaching to the electrodes, bouncing at the electrodes, flying up and down between the electrodes. The breakdown triggered by the microparticles can be grouped into six patterns in terms of the motion of the microparticles before/after the breakdown and the place where the light emission observed. The occurrence probability of each breakdown pattern is also investigated with changing the size of the microparticle. The probability of breakdowns that occur at the collision with the electrode becomes higher with the size of the injected microparticle. The occurrence probability of breakdown that are triggered by an attaching microparticles becomes higher with the time from the voltage application. It is important to ensure the gap not to be contaminated with the big microparticles and/or attaching microparticles.