Multiple shield arrangements breakdown model in vacuum interrupters

Abstract

Switching chambers of vacuum interrupters (VI) are mainly built with ceramics as insulators. During the interruption process, the ceramics are coated by metal vapor from the contact material. In a worst case scenario, an electrical bypass is formed, caused by the metal coating on the ceramic insulators. To avoid this, dielectric shielding is used to provide the vapor coating surfaces with defined potentials. Different shield arrangements offer a balance between local field stress and effective protection against impact from the metal coating. In the current work, we developed an electric field simulation approach using COMSOL software. The purpose was to study the impact of the deposited layers on the performance of the VI with multiple shield arrangements. The simulations were done based on three different cases, using the same VI geometry: VI without metal coating on the ceramics; VI with graded metal coating on the ceramics; Breakdown condition between the head-shield and the metal coating on the ceramics. The identification of the effect of coatings on the electric field inside a VI during lightning impulse voltage stress can help to evaluate the breakdown voltage of the whole arrangement. Therefore, we simulated the redistribution of the electric field strength over the remaining insulated shields.