Estimation of breakdown voltages of surface flashovers initiated from triple junction in SF 6 gas

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, SF 6 gas, and a metal electrode come in contact. Using four kinds of test samples, the flashover voltages V f are measured. The voltages V f become lower than the theoretical voltages V s 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 V c from the voltages V f , and investigated the relationship between the voltages V c and the theoretical values V s . The values of V s /V c , which characterize the properties of the partial discharges, can be shown by the product λS 90% E av P of the following parameters: (1) coefficient λ representing the surface roughness of electrode at the triple junction; (2) effective area S 90% of the electrode; (3) average field E av at the area S 90% ; 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 V s /V c .