Effects of Impulse Voltage Polarity on Surface Leader Discharges in SF6 Gas

The authors deal with the surface discharges initiated from the triple junction at which a solid insulator, SF6 gas and a metal electrode come in contact. The process of the surface discharges at impulse voltages has been measured optically. The voltages of the leader inception and the surface flashover increase with the gas pressure for positive polarity, while they are almost independent for negative polarity. The difference can be explained by the following polarity effect of the scaling laws on the leader transition and the leader propagation. (1) The streamer to leader transition time for positive polarity is inversely proportional to the gradient of impulse voltage dV/dt. The transition time for negative polarity is inversely proportional to the products of the pressure P and the gradient dV/dt. (2) The positive leader propagation speed is described by the products of the leader inception voltage V1d and the gradient dV/dt, while the negative speed is PV1d (dV/dt). The polarity effect can be accounted for by the difference in the pressure dependences of streamer channel radius between positive and negative polarities.