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Motion and Breakdown Related to Microparticles in Vacuum Gap

Vacuum circuit breakers (VCBs) are widely used in the medium voltage class. In VCBs, late breakdowns sometimes occur after the circuit current has been interrupted, 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 the application of dc voltage. It turns out that the motion of microparticles can be grouped into three patterns: attaching to electrodes, bouncing at electrodes, and bouncing up and down between electrodes. Some microparticles gain positive/negative charge during the flight as the acceleration changes despite application of dc voltage. The microparticles are positively charged by the secondary electron emission under the bombardment of field emission electrons from the cathode, while they are negatively charged by absorbing field emission electrons. The patterns of breakdowns triggered by the microparticles can be grouped into six patterns according to the motion of the microparticles before/after the breakdown and the location of light emission. The occurrence frequencies of each breakdown pattern are also measured with changing the microparticle size. It can be said that contamination of the gap with big and attaching microparticles should be avoided in order to prevent degradation of the withstanding voltage of the gap.