Influence of Voltage Waveform and Duty Cycle on Solid Insulation Strength in High Frequency Transformer

High-Frequency Transformers (HFTs) are subjected to different types of voltage stress, including DC or AC grid voltage and high-frequency square wave voltage. As the demand for higher voltage ratings and greater compactness of HFTs continues to rise, the insulation strength of resin cast solid insulation becomes a crucial factor in the insulation design of HFTs. This study investigated the difference in breakdown voltage (BDV) under various voltage types, obtained the BDV of specimens with different installation polarity, and analyzed the influence of duty cycle for unipolar square wave. Additionally, the dynamic behavior of space charge was explained using a 2D axisymmetric bipolar charge transport (BCT) model. The results showed that the BDV is primarily determined by the peak-to-peak field for AC voltages. The unblocked carrier injection in the insulation causes a reduced BDV for AC voltage compared to DC voltage. Furthermore, long-term space charge accumulation has a negative impact on insulation strength under unipolar square wave, as it encourages the enhancement of local current on the electrode. The BDV was observed to reach its maximum value at a duty cycle of 10% for unipolar voltage, due to the balance between the barrier effect and the current enhancement effect of space charge.