Effect of charge accumulation on ac breakdown through a crack between two insulating walls

Breakdown in air through a crack between two insulating plane walls was investigated, using a knife edge, plane electrode configuration. A crack of width s was formed by two plane walls of PE blocks. The minimum crack width studied was 15 μm. The maximum electrode distance studied was 80 mm. The experiments were performed in air controlled at 20 to 25°C and 40 to 50%RH. In the region of the crack width 15>s>100 μm (A region), ac breakdown voltage (BDV) decreased with the decrease in s. In the region 0.1>s>1 mm (B region), BDV slightly increased with a decrease in the width. BDV decreased at the width of the crack where the magnitude of positive charge due to ac partial discharges (PD) was large. The magnitude of the negative charge due to PD was almost unchanged in the B region, but it increased in the A region. The surface voltage distribution on the wall suggested that positive charge accumulation on the wall and its neutralization play an important role of charge quantity of PD on the crack width