Effect of insulator surface temperature on the flashover voltage of outdoor insulators

Contamination on power system insulation can cause flashovers and subsequent outages when moisture deposits on the surface of the insulators to form electrolytes. Fog and light drizzle have been observed to be the most conducive form of moisture leading to flashovers; whereas, rain will wash off most of the soluble contaminants on the insulation's surface. The initial wetting is therefore a critical stage in the flashover process of contaminated insulators. This paper describes flashover tests made in a fog chamber on dc energized suspension insulators. The test insulators were preheated and the effect of the insulator's relative surface temperature on the flashover voltage was noted. The test results indicate that if the insulator's body heat can keep the surface sufficiently dry through this critical wetting stage, there can be a significant increase in the flashover voltage: as high as four times that of an insulator with a surface temperature equal to the air temperature and under the same contamination and fog conditions. This finding could be applied to abate contamination problems on insulation at severe pollution sites. To generate the required heating, an impedance element can be embedded in an insulator with a castable dielectric body. Calculations indicate that the energy necessary to obtain this internal body heating effect will be comparable to the energy losses due to leakage current and partial arcing caused by the contaminant layer.