Prediction of Lightning Impulse Voltage Induced Breakdown in Contact Gap of Vacuum Interrupters

Breakdown voltages of contact gaps are important for the design of vacuum interrupters, especially for high voltage vacuum interrupters. In order to increase the accuracy for insulation design of the vacuum interrupter, breakdown voltage should be predicted to determine the contact gap. The breakdown theory of contact gap in vacuum was unclear. Furthermore, the breakdown of contact gap in vacuum was influenced by many complex factors. By analyses the process and characteristics of the breakdown in contact gap, we proposed a hypothesis that the maximum temperatures were equal at breakdown moment for two anodes which had identical materials and production processes. Base on this hypothesis, we established an expression to associate the breakdown voltage of a contact gap at two different gap distances. We designed a special vacuum interrupter to measure the breakdown voltage for the contact gap at different gap distances. Negative polarity lightning impulse voltage $(1.2\times\ \text{SO}\ \mu \mathrm{s})$ was applied by an up-and-down method. Moreover, the 50% breakdown voltage was used to represent the breakdown voltage. The measured $U_{50}$ were used to calculate and verify the breakdown voltage of the contact gap. Experiment results proved that the $U_{50}$ from this prediction method was very closed to the $U_{50}$ from measurement. The relative error of the calculated $U_{50}$ from this prediction method didn't exceed 6% when the difference between two contact gap distances was small than 3.5 times. Furthermore, the relative error of $U{50}$ from this prediction method was much less than relative error of $U_{50}$ from Slade's prediction method. So that the prediction method is useful for vacuum interrupter design.