Estimation of Breakdown Voltages of Surface Flashovers initiated from Triple Junction in SF₆ Gas

Abstract

In the present report the authors propose the method 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, we have measured the flashover voltages V_f. 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 λ S_90%E_avP 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.