Breakdown characteristics of long air gap with negative polarity switching impulse

Breakdown characteristics of air gaps under negative switching impulse are complicated. To better understand the nonlinear discharge characteristics of long air gaps, the 50% breakdown characteristics of rod-rod gaps, rod-plane gaps and rod-conductor gaps, with air gap distance ranged from 1 m to 10 m, are studied thoroughly, with two waveforms of negative switching surges. Saturating characteristics have been obtained for the above three typical gaps. For air gaps exceed 4 m, the 50% breakdown voltage appears to be lower for rod-plane gaps than that for rod-rod gaps, while the average breakdown electric field decreases with the length of air gaps. Gap factors under negative impulses tend to level off with gap spacing. Furthermore, new knowledge is obtained indicating that, the 50% breakdown voltage for rod-conductor gaps may be lower with conductor grounded than that with conductor ungrounded. Finally, it also indicates that the wave front of the applied voltage has great influence on the breakdown time of rod-rod air gaps.