DC flashover performance of ice-covered insulator strings adopted in ±800kV transmission lines based on up-and-down method

Based on up-and-down withstand method, a new approach and procedure to perform icing flashover test for insulator strings of DC system are proposed and achieved, through which 50% flashover voltage of iced insulators can be evaluated under one constant test voltage. The approach accords with real operation condition of DC system, which is better than methods based on rising voltage to flashover. Tests are carried out in the artificial climate chamber (diameter 22m, height 34m) of Pollution and Environment Lab at China Electric Power Research Institute (CEPRI). Under full-scale conditions (insulator string length 12.5m), icing flashover characteristics of long insulator strings adopted in ±800kV UHV DC transmission lines are obtained. Both in icing and melting phase, 50% flashover voltage and deviation of insulator string are obtained, which shows flashovers occurring at melting phase are more easier than that at icing phase. Comparison between rising voltage to flashover method and up-and-down method is made, which shows advantages of the latter. Under constant test voltage, the distribution of icicles and air gaps along insulator string is relatively fixed. Besides, leakage current is analyzed to explain the flashover or withstand process of long insulator strings under constant DC voltage.