The discharge characteristics of composite post insulator are different under power frequency voltage, operating overvoltage and lightning overvoltage. Generally, only the creepage distance can meet the insulation coordination requirements under power frequency voltage. 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 operating 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 overvoltage waveform of operating overvoltage is closer to that of power frequency voltage. Therefore, the zero value insulator will affect its discharge characteristics. Therefore, when calculating the tension insulator under operating overvoltage, a certain number of zero value insulators must be added to the suspension string. However, due to the short discharge time, the effective string length is mainly considered for lightning overvoltage, and the zero insulator will not reduce the effective string length of insulator string. Therefore, it is not necessary to add zero value insulators in the insulation coordination of lightning overvoltage. Whether it is power frequency voltage, operating overvoltage and lightning overvoltage, the surface dissociation characteristics of insulators are poor due to the influence of air density after the elevation increases. Therefore, altitude correction must be carried out.
1. Power frequency voltage
It only needs to meet the climbing distance, and the creepage method can be used for suspension and tensioning! This method does not need tower height correction or zero value correction (creepage is an empirical value, considering the influence of zero insulator).
2. Lightning overvoltage
When the tower height is less than 40m, the length of insulator string must meet the minimum requirements in table 7.0.2; When the tower height is greater than 40m, it must be corrected according to the tower height. The length of modified insulator string is the length required for lightning overvoltage. The number of insulator pieces to be used can be converted by this length (length divided by the height of the insulator to be used).
3. Operating overvoltage
Meet the minimum requirements of table 7.0.2. It is actually used for verification. Since the multiple of operating overvoltage or the discharge characteristics of insulator under operating overvoltage are not given, the length of insulator can not be calculated first, and then the number of pieces can be calculated. If tension is applied, zero is added. Regardless of the height of insulator string, the equivalent length of insulator string after zero value fault must meet the string length required in this table. This is the minimum string length required, and there is no zero value (therefore, the number of zero values of stress string should be added after various corrections are completed, and it will be zero after correction!). Under plain conditions, the tower height does not affect the discharge characteristics and overvoltage value of insulators under operating overvoltage. Therefore, tower height correction is not required for operating overvoltage.
