Due to the electrostatic adsorption effect of DC lines, the pollution level of DC lines is higher than that of AC lines under the same conditions, and more insulator pieces are required. The insulation level of DC lines is mainly determined by the pollution discharge characteristics of the insulator string. Therefore, there are currently two main methods for selecting the number of insulator pieces:

1. According to the artificial pollution test of insulators, the insulator pollution tolerance method is used to measure the pollution flashover voltage of insulators under different salt densities, in order to determine the number of insulators.        

2. According to operating experience, the creepage distance method is adopted. In general, the creepage distance of DC lines in certain regions is twice that of AC lines.        

Among the two methods, the former is intuitive, but requires a large amount of experimental and testing data, and the results of experimental testing are highly dispersed. The latter is simple and easy to implement, but has poor accuracy. In practical applications, the two are usually combined.