Sparkover behavior of insulator under composite switching impulse/alternating or direct voltage

It is well know that, if air distance is same, the value of sparkover voltage between porcelain insulator and composite silicon insulator has no significant difference for simple switching voltage case. However, in actual transmission lines, both kinds of insulators are energized by existing alternating or direct voltage prior to switching impulse voltage, therefore the sparkover behavior of two kinds of insulators under composite voltages is studied in this paper. The test results show that the switching impulse sparkover voltage has a large reduction for both kinds of insulator when direct or alternating pre-stressing voltage gets to critical value. In direct pre-stressing case, the maximum relative reduction in sparkover voltage for polymeric insulator and porcelain insulator is about 20 percent and 10 percent respectively. For alternating pre-stressing case, the maximum relative reduction in sparkover voltage for polymeric and porcelain insulator is about 12 percent and 6 percent respectively. Those reduction effects depends on pre-stressing voltage amplitudes, polarity between pre-stressing voltage and switching impulse, and triggering angles where the switching impulse is superimposed on the alternating voltage