Observation of pre-discharge phenomena with point-to-plane electrodes in vacuum under AC

Protrusions are generated with random distribution on the contacts surfaces of Vacuum Circuit Breaker (VCB) after arc interruption. They will degrade the vacuum insulation level by Joule Heating Vaporization while withstanding rated voltage. In order to investigate the behavior of protrusions in VCB after interruption, the point-to-plane insulation structure is constructed in the vacuum chamber. And, the pre-discharge phenomena are observed using high speed video camera. It is observed that successive current pulse cascades before the explosion of the point electrode, which results in craters on the surface of the point electrode. Through the analysis of the images, the speed of the particle is estimated to be less than 125m/s. This research also shows the vaporization is non-uniformly distributed in some channels around the point electrode, which can be the possible reason for the later discharge in VCB. Through the computation of the Fowler-Nordheim (F-N) Curve, two patterns of field emission are revealed on the F-N plot, which are the rising and falling pattern. In the voltage rising pattern, the field enhancement factor is 11 times larger than that in the voltage falling pattern; however, the effective field emission area is only 1/1000 of the voltage falling pattern. Moreover, the two patterns are enclosed with the non-field emission half cycle to form a cycle on F-N plot. It is concluded that the point electrode is under periodic conditioning process with the observation of the cycle, which is considered to be the key reason for late discharge in VCB.