Statistical Failure Characteristics of Air Gap Subjected to Damped Alternating Overvoltages and Its Risk Assessment

This paper examines characteristics of the statistical failure of rod-plane air gap (RPAG) under damped alternating overvoltage (DAOV) as their parameters, including the frequency and the damping factor, are varied. The failure probability is calculated by considering the probability density distribution of overvoltage in power systems, and the results show that the failure probability of insulation under the DAOV is higher than that under double exponential impulse waveforms (DEIW), where this shows the risk of the prevalent insulation design. We then propose a 3D model to predict the characteristics of failure that shows that the probability of failure increases with the frequency and the damping factor. We use a 3D analytical formula to propose a method characterized by a failure graph to search for typical parameters of the area of the waveform that poses a high risk to the insulation. The results of a verification experiment show that the risk of failure of the DAOV in the search area of the waveform is significantly higher than those of other DAOV. Both the proposed 3D prediction model and the search area can help improve the design of the insulation, and can be used to apply statistical methods for insulation coordination in power systems.