Prediction Method of the Breakdown Probability for Multiple Vacuum Gaps in Series Considering Dielectric Recovery Under a Lightning Impulse Voltage

The objective of this article is to propose a novel prediction method of the breakdown (BD) probability for multiple vacuum gaps (VGs) in series considering their dielectric recovery under a lightning impulse voltage. Based on the experimental data of the BD probability for each single VG and the calculated voltage distribution of multiple VGs, the BD probability of multiple VGs in series could be calculated by the proposed prediction method. By dielectric tests under the lightning impulse voltage, we see that the calculated BD probability distributions fit well with the experimental results for double VGs and triple VGs in series. Under sphere–plane electrode VGs, the prediction error was approximately 15% in the whole range of BD probabilities. The errors of the 50% BD voltage were 11.4% and 12.9% for double VGs and triple VGs in series, respectively. Therefore, the reliability of this prediction method considering the dielectric recovery was validated. This prediction method could be used to estimate the dielectric strength of multibreak vacuum circuit breaker (VCB) and multiple floating shields inside high-voltage vacuum interrupters.

I. Introduction

At present, vacuum circuit breakers (VCBs) have been widely used at medium voltage levels of 12–40.5 kV due to their benign performances, such as compactness, eco-friendliness, and high dielectric strength. With the development of VCBs to higher voltage levels above 110 kV, the dielectric strength of VCBs needs to be improved significantly [1]. There are two effective ways. First, multiple floating shields are used in the vacuum interrupter (VI), which could improve its inner electric field distribution with a long contact gap [2], as shown in Fig. 1. The second way is to connect several VIs in series to form a multibreak VCB. This could effectively solve the saturation effect of a long vacuum gap (VG) [3]. Both cases could be considered as multiple VGs in series. For multiple VGs, complete breakdown (BD) is induced by the BD of all single VGs [4]. Thus, the dielectric strength should be determined by the BD probability of each single VG [5]–[7]. Fig. 1.

Multiple VGs in series in a typical high-voltage VI.