CALCULATION OF DC AND AC FLASHOVER VOLTAGE OF POLLUTED INSULATORS

The phenomena and the propagation processes of dc and ac arcs on the surface of polluted insulators were observed and investigated by means of a high-speed camera. The time-resolved photographs indicate that there is an obvious distinction between dc and ac flashover processes. Based on the alternating character of an ac arc on the polluted surface, the relationship between its re-ignition and recovery conditions are analyzed. The most important factor is the recovery rather than the re-ignition condition. A formula expressing the resistance of the pollution layer on the insulator is derived. From this, the flashover voltage of a polluted insulator of complex shape can be calculated. A computer program has been developed to calculate the flashover under both dc and ac. Some arc phenomena, such as arc bridging between the sheds or ribs of insulators, the drift of the arc from the insulator surface, and the number of arcs in series before flashover, are taken into account. Much experimentation has been done to determine the flashover voltages of polluted insulators with complex shapes under both dc and ac. Experimental data are in good agreement with the calculated results. © 1990 IEEE