Mechanism of impulse voltages breakdown under non-uniform field in vacuum interrupters

The objective of this paper is to propose a breakdown model based on the Cranberg “clump” hypothesis for an impulse voltage breakdown under a non-uniform field in high voltage vacuum interrupters. It is assumed that the micro-particles of loosely adhering material are instantaneously detached from a parent electrode surface at a critical voltage given by the Cranberg hypothesis. The influence of non-uniform field distribution on the micro-particle charge has been considered. And the voltage rise rate in impulse voltage occurring during the transit time of the micro-particles to the opposing electrodes has also been considered. For an impulse voltage of constant rise rate applied on the micro-particles, the breakdown voltage of a vacuum gap depends upon the 0.74 power of the gap length for the plate-to-plate contacts with contact gaps 10 to 50 mm. Finally, the hypothesis has been validated by the experiments. And the experimental results show that the impulse breakdown voltage was related to a 0.70 and a 0.79 power of the contact gap depending on a different contact surface roughness. So the experimental results support the Cranberg hypothesis in its application to the impulse breakdown under non-uniform field in vacuum interrupters.