Partial discharge behavior and breakdown prediction in SF6 gas

The authors measured the partial discharge (PD) characteristics from their inception to breakdown in SF₆ gas insulated switchgear (GIS) as a function of gas pressure from 0.1 to 0.4 MPa for a needle-plane electrode configuration with three different radii at the tip of the needle: R=70, 250 and 500 μm. Experimental results revealed that PD began to occur in the negative half cycle as AC applied voltage rose. Above the PD inception voltage, the repetition rate n¯₊ of the PD pulse in the positive half cycle increased more rapidly with enhanced voltage than that in the negative half cycle. The authors propose a PD parameter which can be the indicator for predicting breakdown: an unbalance ratio U_r of the repetition rate between positive and negative PD pulses per one cycle AC voltage. It was found that U_r changed from -1.0 to +1.0 over the applied voltage range from the inception to breakdown. As the applied voltage approached the breakdown voltage, U_r proved to fall from +1.0 to +0.5. The authors also measured changed PD characteristics with time to breakdown for a needle with R=15 μm by AC long term voltage application. U_r was found to decline with approach to breakdown. Thus, they suggest the possibility that U_r could be used as a breakdown prediction parameter in GIS. The mechanism of PD behavior was also discussed