Numerical Evaluation Methodology for Power Electronic Transformer Insulation Based on Space Charge Characteristics Under Combined HF Electrothermal Stress

Power electronic transformer insulation systems endure extended periods of operation under challenging conditions characterized by the interaction of electrical and thermal factors. Current numerical models of space charge inadequately replicate the charge evolution in actual operational circumstances. In this paper, a high-frequency heating model is constructed based on the dielectric loss mechanism, and an improved bipolar charge transport model with coupled frequency-temperature factor is further proposed. Based on the established model, the impact of the temperature-frequency factor on the space charge characteristics is investigated. The results indicate that the extent of charge migration and accumulation diminishes considerably as the stress frequency increases. At a frequency of 1 kHz, the minimum migration depth is merely 6 mu m. Furthermore, the charge accumulation is positively correlated with the polarization power and field strength. Simultaneously, the accumulation charge polarity exhibits distinct properties when subjected to unipolar and bipolar polarization circumstances. The accumulation of heteropolar charges results in heightened distortions in the electric field near the interface. Ultimately, the accuracy of the model is verified by experimental results of space charge at various frequencies and temperatures. This study contributes to the understanding of the space charge evolution and insulation failure mechanisms under complex stresses.