Impulse Partial Discharge Characteristics and Their Mechanisms under Non-uniform Electric Field in N rSF

() ( ) We have investigated impulse partial discharge PD and breakdown BD characteristics of a needle-plane gap in N r r rSF gas mixtures under positive lightning im26 pulse voltage application, and discussed their physical mechanisms. The 50% () ( ) probability PD inception voltage PDIV , leader discharge onset voltage LOV 50 () and BD voltage BDV were measured and analyzed as a function of gas pressure 50 and SF content. Experimental results revealed the stepwise propagation process 6 of the impulse PD and enabled us to classify the impulse PD in N rSF gas mix26 tures into two types, the streamer discharge and the leader discharge. We also discussed the impulse PD propagation mechanisms in terms of PD parameters such as propagation length, time interval and current pulse magnitude, and suggested a sequential relationship in the PD propagation process under non-uniform electric field.