Evaluation of overshoot rate of lightning impulse withstand voltage test waveform based on new base curve fitting methods - application to practical diverse waveforms

In a lightning impulse withstand voltage test for large-sized electric power equipment, voltage waveforms sometimes contain overshoot due to the capacitance of the test equipment and the residual inductance of the test circuit. The authors have been studying fitting methods for extracting a reasonable base curve from this overshoot waveform and constructed a new base curve extraction method. In this paper, a verification study was performed using diverse overshoot waveforms to show the rationality of this new method from a more practical perspective. To be specific, the study used actual waveforms with the frequency of the overshoot part varied as well as waveforms with the overshoot rate less than 10%. Consequently, it was found that the new method was a more reasonable one to calculate the overshoot rate of several to several tens of percent which is an actual range for evaluation in the test. Meanwhile, it should be noted that the k-factor filtering scheme was implemented after the application of various fitting methods to test data generator waveforms, and then the shape parameters of the test voltage waveforms, such as the crest value, front time, and time to half-value, were almost identical, which confirmed that fitting methods had only a minor influence on the test voltage waveform. Furthermore, assuming an actual test for UHV-class equipment, the relationship between the front time and the overshoot rate was investigated in detail. As a result, it emerged that, although it was difficult to generate waveforms complying with the present standard if the capacitance of the test equipment was large, the number of equipment that could comply with the standard would significantly increase if the value of standard for the front time was relaxed.