Estimation of breakdown voltages of surface flashovers initiated from triple junction in SF6 gas

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Abstract
In this paper, a method is proposed to estimate the minimum breakdown voltages of surface flashovers initiated from the triple junction at which a solid insulator, SF6 gas, and a metal electrode come in contact.Using four kinds of test samples, the flashover voltages Vf are measured. The voltages Vf become lower than the theoretical voltages Vs of streamer inception in the case of the test sample with the large electrode area at the triple junction. To understand the phenomena quantitatively, we estimated the real partial discharge voltages Vc from the voltages Vf, and investigated the relationship between the voltages Vc and the theoretical values Vs. The values of Vs/Vc, which characterize the properties of the partial discharges, can be shown by the product λS90%Eav P of the following parameters: (1) coefficient λ representing the surface roughness of electrode at the triple junction; (2) effective area S90% of the electrode; (3) average field Eav at the area S90%; and (4) gas pressure p. The minimum surface flashover voltages in the insulators for coaxial electrode systems can be estimated precisely, considering the properties of Vs/Vc.