Internal breakdown voltage of vacuum interrupters with shield potential control

Summary

Internal breakdown voltage of vacuum interrupters (VIs) is primarily determined by electric field strength and effective area on electrode surfaces. Generally the breakdown in vacuum begins with field emissions from critical enhancement points with higher electric field. The most critical point of the macroscopic electric field in a VI is present at the front contact edge between the main contacts. A vapor shield of a VI is one component that determines the electric field strength and distribution. Its floating electrical potential can basically be determined by the geometry of the VI. Since the shield potential can change the maximum electric field strength at the front contact edge in VI, it seems possible to increase the internal dielectric strength of the VI by giving the vapor shield a well-defined electric potential. The present study focuses on the possibility of electrical potential control of the vapor shield for enhancing the internal withstand voltage of VI. Basically, the potential control of a vapor shield can be realized by an external circuit with parallel capacitors. This can allow controlling the shield potential arbitrarily between H.V. potential and zero by changing the values of the voltage division ratio. From the field calculation results for a 72.5 kV VI model, the electric field strength can be reduced by nearly 20% compared to a freely floating vapor shield. Dielectric withstand tests of the 72.5 kV test VI with potential control to determine the internal breakdown voltage with positive impulse voltage are carried out. The experimental results show that the breakdown voltage can be increased by 11.2%, by setting the vapor shield potential to one third of the applied voltage. The increase in breakdown voltage can be explained by the reduction of electric field strength on the critical point as a result of the decrease in potential difference between cathode and vapor shield.

Additional informations

Authors

VIBHAKAR, GACEK

Keywords

vacuum, electrical breakdown, vapor shield, potential control