Study of vacuum circuit breaker performance and weld formation for different drive closing speeds for switching capacitive current

Experiments have been conducted to study the influence of the closing speed of the drive mechanism on the performance of vacuum interrupters used for capacitive switching. Given the experimental conditions (no load-opening), it was found that the probability of breakdown is about 30% less with a fast closing speed (contacts closing on average before a half-cycle of the inrush current) compared to a slow speed (contacts closing on average after a half-cycle of the inrush current). However, the probability of breakdown depends strongly on the contact history. In particular, it was shown that the weld formed during the inrush current phase exhibits a strong macroscopic deformation of the contact surface that is more pronounced for slow closing speed when the arc duration is the longest and the heat transfer to the electrodes is the highest. This deformation produced, at the end of the set of experiments, a local electric field that was enhanced by up to 3.5 times compare to that of the original surface. Contact cross-sections show that the weld is formed by mass transferred from one contact to the other and by further melting of this additional material after its integration.