Natural contamination test of insulators with DC voltage energization at inland areas

Results of tests for natural contamination of insulator strings in inland areas are discussed. Natural exposure tests with -280 kV DC voltage energization were carried out for 5 years. The following results were obtained: (1) The amount of contaminant on the bottom surface increased with the length of the exposure period, for the first year, at inland areas. Therefore, estimates of contamination severity should use data obtained for exposure periods of at least one year. (2) Insulators near the line and ground sides of a string collect more contaminant than the middle when DC voltage is applied. However, the effect of nonuniform contamination on DC withstand voltage is not as great as expected when ratios of the number of heavily contaminated insulators to that of insulator string are from 15% to 30%. (3) No significant effect of DC voltage energization on the ESDD of the middle part of insulator strings has been observed when the applied voltage is no higher than 280 kV DC. (4) A pollution severity map established for AC transmission lines can be used for the determination of design ESDD for HVDC transmission lines for inland areas without industrial contamination. (5) The maximum value of leakage current under natural conditions is less than 3 mA, and no flashover has been experienced during these 5 years for electrical stress from 90 to 120 kV/m. The application of these results to insulation design for HVDC transmission lines is discussed