A detailed study on the quality assessment of insulators subjected to D C voltages

The following are the conclusions that can be derived based on the result of voltage distribution tests and thermal runaway tests on DC insulators. 1.) Voltage shared by the line end insulator varies inversely with respect to variation of relative humidity. This may be due to the fact that the flow and drain of the surface current reduces due to reduction in relative humidity. This behaviour is similar in the entire insulator except the 2^nd insulator from line end and 5* from line end. The maximum voltage shared in the string is at ground end which may change with respect to RH. This gives the importance of voltage distribution which may be the cause of single unit flashover, pollutant accumulation and the failure due to ion migration and thermal runaway. 2.) In order to assess the quality of insulators, for DC applications, thermal runaway tests were carried out at higher temperatures of 120degC and 150degC instead of 80degC, which was stipulated in the standards. Glass insulators failed at thermal runaway test at 120degC and 150degC. These failed insulators were drawing higher currents. Three of the AC insulators (Porcelain) were drawing higher currents during the test.