Experimental Investigation on the Formation Mechanism of the Lowest Breakdown Voltage Area in a Long Air Gap Containing a Floating Conductor Under Positive Switching Impulses

Long air gaps containing a floating conductor are common in transmission and substation projects. As the floating electrode moved across the gap, a lowest breakdown voltage area (LBA) can be observed. In this paper, an experimental platform for observing and measuring discharges in rod-rod-plane air gaps is used to investigate the discharge mechanism in the LBA. The results indicate that the existence of the LBA is related to the minimum voltage for the discharge channel formation in sub-gap 1 (facing the high voltage rod) and the breakdown voltage of sub-gap 2 (facing the grounded plane). When the floating electrode is closer to the high-voltage electrode than the LBA, the breakdown voltage is increased because the potential of the floating electrode does not reach the breakdown voltage of sub-gap 2 at the time the discharge short circuits sub-gap 1. When the floating electrode is further away from the high-voltage electrode than the LBA, the breakdown voltage is increased because the discharge channel formation in sub-gap 1 requires a higher voltage. The LBA corresponds to the area where these two different phenomena can happen with minimum voltage. The results can contribute to modeling the discharge of a long air gap containing a floating conductor.