Effect of high-frequency pulse voltage parameters and DC bias on breakdown of epoxy cast insulation

The epoxy cast insulation in high-frequency transformers 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 was investigated. Epoxy encapsulated samples were employed in tests to reflect the intrinsic insulation characteristics without partial discharge erosion effect. Meanwhile, dielectric heating was simulated and proved to be far below the thermal breakdown threshold. New patterns were observed compared to other breakdown studies using traditional methods, for example, a slight increase of BDV was 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 was 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 was proposed, describing the non-uniform distribution of interface charge. Main experimental trends were 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.