Breakdown Characteristics of Vacuum Gaps with Geometrically Similar Structures under Lightning Impulse Voltage

In the insulation design inside the vacuum interrupters, a traditional method is to evaluate the dielectric strength through the experiments. For the design of the vacuum interrupter with a large size, the experimental method requires lots of cost and time. The objective of this paper is to determine the breakdown characteristics of the vacuum gaps with geometrically similar structures under the lightning impulse voltage. A negative standard lightning impulse voltage was applied to a pair of copper sphere-plane electrodes. The diameter of sphere electrode varied from 14 to 30 mm. The vacuum gaps with geometrically similar structures were achieved by keeping the ratio of the diameter of sphere electrode to the gap distance at 9.0. The results revealed that, when the sphere diameter varied as 14, 18, 24 and 30 mm, the 50% breakdown voltage of the vacuum gap with a geometrically similar structure was 62.84, 71.46, 92.13, 116.6 kV, respectively. With increasing the size of the similar structure, the maximum electric field strength decreased, while the geometric enhancement factor was nearly constant. The breakdown voltage of a large-sized vacuum gap could be well estimated by using the breakdown field strength of a small-sized gap with a similar structure.