부산대학교 도서관

In vacuum circuit breakers, restrikes and nonsustained disruptive discharges are sometimes observed even up to several seconds after current interruption. These late breakdowns are triggered by a sudden increase in the field emission level and/or microparticle collision with the electrode during the application of recovery voltage. In this paper, we identify the late breakdowns triggered by the microparticles that emerged from the silver-tungsten carbide electrodes after current interruption. The combined use of a high-speed video camera and a pulse laser enabled us to conduct shadowgraph photography with a 150-ns time resolution, 10- $\mu \text{m}$ spatial resolution, and $\mu \text{s}$ framing rate for capturing the microparticle motion under the recovery voltage after current interruption. In this highly resolved photography, we observed many microparticles flying between the electrodes after current interruption. In our series of observations, we found two types of microparticle-inducing late breakdowns: those induced by small, fast microparticles whose diameter and velocity were approximately 20– $40~\mu \text{m}$ and 15 m/s, and those induced by large, slow microparticles whose diameter and velocity were approximately $100~\mu \text{m}$ and 1 m/s, respectively. Some of the large slow microparticles caused the sequential late breakdowns during the up-and-down motion between the electrodes. All the particle-induced late breakdowns occurred at the time of particle collision with the negative electrode defined by the polarity of the recovery voltage. The velocity of the large, slow microparticles was approximately 1 m/s, and these microparticles did not evaporate. Furthermore, the gradual decrease in the particle size was not observed in the continual occurrence of late breakdowns. We also observed many breakdowns caused by unknown factors. The electrode surfaces were also observed by using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The microstructures and metal compositions on the electrode surfaces varied drastically with the observation positions.