The insulation performance and the flashover mechanism of a vacuum gap bridged by an insulator in the shape of a conical frustum have been studied. The cone angle of the insulator is varied from 0 to 45 degrees, and the thickness of the insulator is varied from 5 to 15mm. The gap is subjected to a lightning impulse voltage of ±2/50μs. The insulation performance is investigated by observing the ratio of the flashover voltage of the bridged gap to that of the gap without insulators. The flashover mechanism is investigated by observing the distribution of traces on the cathode due to flashovers and by analyzing the electric field near the insulator-cathode junction. Charging of the insulator due to electron collision on its surface is considered in this analysis. As a result, it is found that the ratio is greater than 90% for a cone angle greater than a critical one, when the top of the frustum is subjected to the positive impulses. It is also found that the trace distribution is closely related to the insulation performance.
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