External Dielectric Performance of Various Vacuum Interrupter Envelope Constructions

A vacuum interrupter envelope consists of metal parts such as end caps in addition to the ceramic insulator and sometimes an external metal center shield. In this work envelopes of identical ceramic insulator height with various metal component constructions were tested for their external dielectric performance in a design-of-experiments (DOE), using experimental vacuum devices containing an internal pair of specially built half-sphere electrodes with a very large vacuum gap, maximally seasoned to ensure that breakdowns under test voltages are external. Effects of the height of the seal-cup and/or the external center shield, as well as the testing setup surrounding the tube were examined. Comparisons were also made between the flat-joined copper seal-cup and the knife-edge-joined dome-shape steel seal-cup. Results were discussed with the objective to identify key parameters for the external dielectric performance of a commercial vacuum interrupter under different application situations.