Influence of Current Interruption on V-t Characteristics of Vacuum Interrupters

To identify the influence of current interruption on the insulation strength of vacuum interrupters (VIs), the voltage to time-lag $(V{-}t)$ characteristics of VIs are measured from nanoseconds to several microseconds range before and after current interruption. The breakdown voltage increases after the current interruption for VIs with Cu–Cr contacts. The field emission current of contacts are also measured before and after the current interruption. From the results and using Fowler–Nordheim analysis, the field enhancement factor $\beta$ and the emission area $A$ are investigated. The analysis shows that the surface condition is improved by the current interruption, which can explain the improvement of breakdown voltage, and that the impulse voltage application during the $V{-}t$ characteristics measurement has a considerable influence on the surface condition. Through the cross-sectional observation of the contact surface using scanning electron microscopy, it also turns out that a fine Cr layer is formed by the current interruption and it might increase the breakdown voltage. In addition, the lower envelopes of $V{-}t$ characteristics are estimated based on the cathode-induced breakdown model and the calculated curves agrees well with the experimental curve.