Due to the electrostatic adsorption of DC lines, the fouling level of DC lines is higher than that of AC lines under the same conditions, and the number of insulator pieces required is also higher than that of AC, whose insulation level is mainly determined by the fouling discharge characteristics of the insulator strings. Therefore, there are two main methods in selecting the number of insulator pieces:
1. the insulator fouling test is carried out using the insulator fouling withstand method, which measures the fouling flash voltage of insulators at different salt densities, thus determining the number of insulator pieces.
2. the creepage ratio method is used in accordance with operational experience, and the creepage ratio of DC lines in general areas is twice that of AC lines. Of the two methods, the former is intuitive, but requires a large amount of test and inspection data, and the results of the test and inspection are widely scattered. The latter is simple and easy to use, but less accurate. In practice, a combination of the two is usually carried out