Influence of Middle Gap Distance on Breakdown Characteristics of Triple Vacuum Gaps in Series

The objective of this paper is to determine an influence of middle gap distance on the breakdown characteristics of triple vacuum gaps in series under the lightning impulse voltage. In the experiments, the triple vacuum gaps were built up by connecting three sphere-plane gaps in series. The diameters of the sphere electrode and the plane electrode were 18 and 50 mm, respectively. The middle gap distance d
m
varied from 0.5 to 3.0 mm and the other two gap distance were fixed at 1.0 mm. The material of the electrodes was Cu. A negative standard lightning impulse voltage was applied with the up-down method. The results showed that, when d
m
increased and the distances of the other two gaps were fixed, the voltage distribution across the triple vacuum gaps remained nearly unchanged. Only the maximum field strength of the middle gap decreased significantly. The 50% breakdown voltage U
50
of the triple vacuum gaps increased from 76.06 to 139.8 kV as d
m
increased from 0.5 to 3.0 mm. With the increase of d
m
, the growth rate of U
50
first increased, and then decreased after d
m
reached 2.0 mm.