Phase relationship of PD pulses in dielectric liquids under ac conditions

Partial discharge pulse phase analysis carried out on a mineral oil and a p erfluoropolyether with point-to-plane electrodes provided further and addit ional evidence that discharge epochs tend to be concentrated in the vicinit y of the alternating voltage peaks at discharge inception in lieu of the vo ltage zeros characteristic of normal cavities occluded in solid and liquid- impregnated solid insulating systems. Relatively large positive polarity di scharge pulses, having an associated charge transfer of greater than or equ al to 15 pC with a recurrence rate of at least one per every ten consecutiv e cycles, appear initially on the positive half-cycle; with further increas es in applied voltage both their number and magnitude continue to exceed th ose of negative polarity over the negative half-cycle. This behavior is to be distinguished from that commonly observed with high sensitivity measurem ents, which clearly indicate that early discharge onset in liquids is chara cterized by the occurrence of minute highly intermittent negative polarity pulses. The increase in charge transfer of the discharge pulses with applie d voltage is suggestive of the development of longer more intense streamers in the dielectric liquids at the more elevated electrical fields. Also the results infer that discharges or streamers form and propagate more readily in the mineral oil than in the electronegative perfluoropolyether.