Motion and production of microparticles in vacuum interrupter

Vacuum circuit breakers are widely used in medium-voltage class. In vacuum circuit breakers, restrikes and non-sustained disruptive discharges sometimes occur after current interruptions. It is said that these late breakdowns are triggered by the microparticles. This paper aims to investigate the relationship between the motion of microparticles and subsequent late breakdown phenomena. It does by artificially injecting microparticles into the vacuum gap and observing their motion using a highspeed video camera. We focused on the sputtering of the electrode material during the current interruption and the mechanical stress of the switching operation as being the origin of these microparticles. We found that the motion of microparticles can be grouped into two distinct patterns: bouncing and attaching. Bouncing microparticles seem to be made of metal, while most of the attaching microparticles attached themselves to the cathode, and their charge was about five times greater than that of the bouncing microparticles. The attaching microparticles seem to be positively charged by secondary electron emissions from the surface under electron bombardment due to the field emission current from the cathode. From these results, we believe that the surfaces of the attaching particles are covered with an insulating layer. From our observations of the production of microparticles by mechanical stress, we have found that the breakdown voltage of the gap drastically drops after a mechanical switching operation where microparticles are produced. The breakdown voltage is restored with repetitive mechanical switching operations and subsequent voltage application. However, the breakdown voltage drops again when the operating speed of the electrode is increased. We also found that microparticles are scattered from the cathode spots on electrodes that have not been current-conditioned, and they are hardly produced from electrodes that are current-conditioned. From these results, we believe that controlling the speed of the mechanical switching operation and the current conditioning are effective ways to prevent the microparticles from being produced.