부산대학교 도서관

The recovery characteristics of the withstanding voltage of a vacuum gap are crucial for the development of direct circuit breakers with mechanical switches. In the active-current-injection-type dc circuit breakers, a negative voltage, i.e., a voltage of polarity opposite to that of the interrupting current, is initially applied after current zero caused by the energy stored in the active-current-injection branch. Subsequently, the recovery voltage obtains the same polarity as the interrupting current, namely, it becomes positive as the capacitance around the mechanical switch is charged. In this study, the recovery characteristics of the withstanding voltage for the negative and positive polarity are measured by applying impulse voltage after the current zero of dc interruption simulating current. The recovery of the withstanding voltage for a positive impulse takes a longer time than that of the negative impulse. The recovery of the positive withstanding voltage is determined by the decrease in the temperature of the microprojections on the cathode spot edges. The postarc current analysis shows that the postarc plasma decays with time according to the power law in the time range of 1–50 $\mu \text{s}$ after current zero. Under the dc interruptions, where the current decreasing ratio increases, the residual plasma density after current zero tends to be higher than that under ac interruptions.