The discharge characteristics of composite post insulators are different under power frequency voltage, operating overvoltage and lightning overvoltage. Generally, only the creepage distance can meet the requirements of insulation coordination under power frequency voltage. After all, the power frequency voltage is applied to the insulator for a long time. The discharge characteristics of operating overvoltage and lightning overvoltage are mainly affected by the length of insulator string. The operating overvoltage is the internal overvoltage, so the influence of tower height is not considered, and the lightning overvoltage is the external overvoltage, and the tower height must be considered. In addition, the overvoltage waveform of the operating overvoltage is closer to the waveform of the power frequency voltage.
Therefore, zero-value insulators will affect their discharge characteristics. Therefore, when calculating the tension insulator under the operating overvoltage, it is necessary to add a certain number of zero-value insulators on the overhang string, but due to the short discharge time, the lightning overvoltage mainly considers the effective string length, and the zero insulator will not reduce the effective string length of the insulator string, so it is not necessary to add zero-value insulators when carrying out the insulation coordination of lightning overvoltage. Whether it is power frequency voltage, operating overvoltage or lightning overvoltage, the surface dissociation characteristics of the insulator are poor due to the influence of air density after the elevation is increased. Therefore, elevation correction is necessary.
More specifically:
1, power frequency voltage
Just meet the climb distance, and the creepage method can be used for suspension and tensioning! This method does not require tower height correction or zero value correction (creepage is an empirical value that takes into account the effect of zero insulators).
2, lightning overvoltage
When the height of the pole is less than 40 meters, the length of the insulator string must meet the minimum requirements of Table 7.0.2; When the height of the tower is greater than 40 meters, it must be corrected according to the tower height. The length of the 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 in Table 7.0.2. It's actually for verification. Since the multiple of the operating overvoltage or the discharge characteristics of the insulator under the operating overvoltage are not given, the length of the insulator cannot be calculated first, and then the number of pieces can be calculated. If tension is applied, zero is added. Regardless of the height of the insulator string, the equivalent length of the insulator string after the zero value fault must meet the string length required in this table. This is the minimum string length required, with no zero value (so the number of zeros in the stress string should be added after various corrections are completed, and the correction is zero!) . Under plain conditions, the height of the tower does not affect the discharge characteristics and the value of the insulator under the operating overvoltage, so it is not necessary to correct the tower height for the operating overvoltage.
