Statistical Characteristics of Breakdowns in Long Air Gaps at Negative Switching Impulses

Discharge characteristics in long air gaps follow certain statistical regularities. Statistical methods and finite element simulation were both employed to deeply study discharge characteristics. The statistical results indicate that the height of grounding rod has a slight influence on the 50% breakdown voltage of rod-rod air gaps and that the breakdown time is longer than the wave front time when 20/2,500 mu s switching impulse is applied and shorter than the wave front time when 80/2,500 mu s switching impulse is applied. Furthermore it can be shown that the distribution of the breakdown voltages in 10 m rod-plane gap follow the normal distribution and that the V-t characteristics of rod-rod gaps is lower than that of rod-plane gaps. Calculation results indicate that the non-uniform coefficient of the electric field increases with increasing gap distance and that the ratio of gap distance to the non-uniform coefficient also increases with the gap distance. The work described in this paper can be used for the insulation coordination of UHV transmission lines.