Effect of non-uniform pollution on the withstand characteristics of extra high voltage (EHV) suspension ceramic insulator string

Pollution flashover of line insulators may cause regional accidents. Research on the effect of non-uniform pollution on the electrical performance of insulators is important for the outdoor insulation coordination. A study on the effect of the non-uniform pollution distribution on the withstand characteristics of an insulator string with 14-unit standard XP-70 ceramic insulators for 220 kV transmission lines is presented. Test results show that ??_ESDD, N and T/B have their own effects on the withstand voltage of polluted insulator string. And their effects are independent from each other. The withstand voltage of the insulator string increases with the decrease of T/B. By regressing the test results, the relationship between the withstand voltage of 14-unit XP-70 insulator and ??_ESDD, N as well as T/B is obtained, which can be used in the outdoor insulation coordination of transmission lines. Moreover, the mechanism of the effect of non-uniform pollution distribution on the withstand voltage is analysed. The non-uniform pollution distribution along the insulator surface causes the decrease of the equivalent surface conductivity and the difference of the hydroscopicity between the top and the bottom surfaces, which results in the increase of the withstand voltage and the decrease of the mean root-mean-square value of leakage current.