The dynamic measurement of surface charge distribution deposited from partial discharge in air by Pockels effect technique

A recently developed electro-optical Pockels effect technique, which allows the dynamic measurement of surface charge distributions, is employed to observe the dynamic behaviour of a partial discharge in a point - dielectric gap. One period of an 8 kV ac voltage is applied to a needle electrode with gap spacings ranging from 0 to 7 mm. Different discharge behaviour and different deposited surface charge distribution patterns are observed for different point - dielectric gap spacings. The electrode gap spacings can be roughly divided into three regions, namely the surface discharge region, the transition region and the space discharge region. In the transition region, both surface discharges and space discharges which consist of a spark discharge and corona discharge are observed. The memory effect associated with the accumulated surface charges plays an important role in controlling the discharge behaviour. The surface charge field component may either suppress or enhance the local field near the point electrode depending on the phase of the applied voltage. `Back-discharges' are the typical result of the memory effect. Negative surface charge deposited from previous discharges can have a considerable influence on the propagation of the subsequent positive surface streamer channels depending on whether they develop in the surface discharge region or in the transition region.