Measuring External Dielectric Strength of the Vacuum Interrupter Envelope

Abstract

Widening application of the vacuum interrupter in SF6 or solid encapsulation demands the need for better understanding of its external dielectric behavior. A device with a pair of fixed half-sphere stainless steel electrodes built into a commercial vacuum interrupter envelope is developed to ensure that breakdowns under high voltage are almost always external rather than internal. Ceramic envelopes of various diameters and lengths are tested in such a method, and the improvement effects of different external insulation media are measured. It is observed that, just like internal breakdowns in vacuum, external flashovers are also statistical in nature, Basic Impulse Level (BIL) and Power Frequency Withstand (PFWV) testing have the healing effect, and external seasoning may be beneficial. Even though adding ceramic length helps to raise the external dielectric strength, its effectiveness decreases as the length goes up. The presence of a center joint in a double-ceramic tube weakens the dielectric strength to lower than that of a single ceramic of the same length. Surrounding the test tube in compressed air is more effective than shrink-wrapping or polyurethane potting in improving the BIL, but perhaps not so on the PFWV, for which shrink-wrapping is apparently but not significantly better than the others. Parameter optimization of the production potting process on any given ceramic envelope can be achieved via such a testing vehicle.