Increasing the Internal Field Strength of Vacuum Interrupters With Vapor Shield Potential Control.

Internal dielectric strength of vacuum interrupters (VIs) is primarily determined by the macroscopic electric field strength and effective area on the electrode surfaces. While internal dielectric strength is usually not an issue for medium-voltage VIs ($U_{m}$ ≤ 52 kV), it requires special consideration when going to higher voltage levels. Generally, the vacuum breakdown is initiated by field emissions from critical enhancement points with higher electric field on a cathode. 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 and its surroundings. Since the shield potential can change the maximum electric field strength at the front contact edge in the 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 increasing the internal withstand voltage of the VI. Basically, the potential control of the vapor shield can be realized by connecting a parallel capacitor network. This allows controlling the shield potential arbitrarily between H.V. potential and zero by changing the voltage division ratio. From the field calculation results for a 72.5 kV VI model, the maximum electric field strength can be reduced by up to 23.3% compared to a floating vapor shield. Dielectric withstand tests for the 72.5 kV VI with potential control to determine the internal breakdown voltage with positive impulse voltage were 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 the breakdown voltage of the VI can be explained by the relaxation of the electric field on the critical point as a result of the decrease in the potential difference between cathode and vapor shield. [ABSTRACT FROM AUTHOR]