Breakdown of shield gaps in vacuum interrupters

The objective of this paper is to determine how the high-voltage discharging metal vapour deposited on the nearby ceramic inner surface influences the breakdowns of gaps between centre shield and end shield in vacuum interrupters. Two types of shield materials were selected, namely copper and stainless steel. The end curvature radius of the shield was 3 mm. The distance between the shields could adjust manually from 4 to 8 mm. The distance between the shields and ceramic was 3.5 mm. The negative polarity standard lightning impulse voltage (12/50 mu s) was repeated for 900 operations based on an up-and-down method. The experimental results illustrated that the metal deposition layer on the inner surface of the ceramic envelope significantly influenced the breakdown voltage of the shield gaps. At a shield gap distance of d = 4 mm, the breakdown voltage of the shield gap increased from an initial lower voltage to the saturation voltage by approximately 200 operations. Then, the breakdown voltage decreased to a lower voltage range during only 50 operations, and the breakdown voltage maintained at this lower voltage range until the end of 900 operations. Furthermore, as the shield gap distance increased from d = 4 to 6 mm and 8 mm, the breakdown voltage also maintained at this lower voltage range. A metal deposition layer was formed on the inner surface of ceramic by repetitive application of the lightning impulse voltage. To analyse the influence of the metal deposition layer, the electric field distributions were calculated for the original vacuum interrupter and the vacuum interrupter with the metal deposition layer on the ceramic inner surface. The simulation results suggested that the metal deposition layer took part in the breakdown path between the shield gaps and deteriorated the insulation performance.