Effect of High-Frequency Pulse Voltage Parameters and DC Bias on Break- down of Epoxy Cast Insulation

The epoxy cast insulation in high-frequency transformers (HFTs) withstands complex inverter-type voltage stresses. In this work, the influence of voltage type, pulse voltage parameters, and dc bias on breakdown voltage (BDV) of epoxy cast insulation is investigated. Epoxy encapsulated samples are employed in tests to reflect the intrinsic insulation characteristics without partial discharge (PD) erosion effect. Meanwhile, dielectric heating is simulated and proved to be far below the thermal breakdown threshold. New patterns are observed compared with other breakdown studies using traditional methods; for example, a slight increase of BDV is found with increasing pulse voltage repetition frequency, indicating the existence of mechanisms other than the per-pulse aging. The avalanche process triggered by local charge variation is concluded to be the first cause of high-field breakdown. To simplify the analysis of dynamic charge behavior with inverter-type voltages, an equivalent model is proposed, describing the nonuniform distribution of interface charge. Main experimental trends are explained by implementing the maximum space charge current level as an indicator of breakdown. This work provides new insights into the aging and failure mechanism of polymer insulation under inverter-type waveform.