Lifetime Estimation of Epoxy Cast Insulation under Medium-frequency Square Voltage with Ramp Breakdown Tests

The medium-frequency (MF) square voltage stress generated by power modules challenges the integrity of epoxy resin cast insulation in MF transformers (MFT). This study aims at a lifetime prediction by ramp stress tests with MF square voltages in the absence of partial discharges. Weibull-inverse power model was employed for the breakdown voltages (BDVs) with modulated voltage ramp rates. The obtained inverse power terms are 11.04 and 14.29 for 1 kHz and 10 kHz unipolar voltages, indicating a high-field per pulse aging mechanism. The non-monotone trend of BDVs with different voltage ramp rates for 10 kHz voltage were found and discussed.