The discharge characteristics of insulator are different under power frequency voltage, operation overvoltage and lightning overvoltage. Generally, the insulation coordination under power frequency voltage only requires the creepage distance to meet the requirements. After all, the power frequency voltage is applied to the insulator for a long time. The discharge characteristics of switching overvoltage and lightning overvoltage are mainly affected by the length of insulator string. Among them, the switching overvoltage is internal overvoltage, so the influence of tower height is not considered, while the lightning overvoltage is external overvoltage, so the tower height must be considered. In addition, the over-voltage waveform of the operating over-voltage is closer to the waveform of the power frequency voltage, so the zero value insulator will affect its discharge characteristics, so when calculating the tension insulator under the operating over-voltage, a certain number of zero value insulators must be added more than the suspension string; while the lightning over-voltage mainly considers the effective string length due to the very short discharge time, and the zero value insulator will not be reduced The effective string length of insulator string, so it is unnecessary to add zero value insulator in the insulation coordination of lightning overvoltage. Whether it is power frequency voltage or operation overvoltage or lightning overvoltage, the de dissociation characteristics of insulator surface will be worse due to the influence of air density after the elevation is increased, so the elevation must be corrected.