Optimization of Insulation Material Performance Parameters for Medium Frequency Transformers Based on Thermosolid Coupling

Power electronic transformers are the core equipment in the future smart grid and energy internet. In order to meet the needs of the grid, the voltage level of power electronic transformers will be increased to 35 kV, and the capacity will also reach 200 MW and above. Working in a high frequency, high voltage, and high temperature environment, the core component of a power electronic transformer, a medium frequency (MF) transformer, becomes the weakest link. Insulation of MF-transformers by casting epoxy resin is one of the most competitive solutions with small size, maintenance-free, clean, and environmentally friendly. However, there are still some problems with epoxy resin casting, such as heat dissipation difficulties, partial discharge, and insulation cracking. This paper optimizes the performance parameters of the insulation materials to match to improve the heat dissipation and mechanical performance of the MF-transformer. Firstly, a finite element model of resin-cast MF-transformer with forced air cooling is built in COMSOL. Simulations were performed according to the requirements of the transformer temperature rise test and thermal shock test. The temperature field during the rated operation and the stress field and displacement field during the thermal shock test were analyzed, and evaluation indexes were given according to the characteristics of insulation materials. The glass transition temperature of epoxy resin is generally lower than the permissible temperature of its heat-resistant grade. Thus, the glass transition temperature is used as the standard for evaluating the temperature rise of resin-cast IF transformers. In addition, the stress of the insulation layer in the thermal shock test needs to be less than the tensile strength of epoxy resin, and the offset of the insulation layer needs to be less than the elongation at the break of epoxy resin. The simulation results show that the pure epoxy resin cannot meet the temperature rise of the transformer in rated operation and the stress requirements in the thermal shock test. Then, the influence of insulation material performance and parameters on transformer temperature rise, stress, and deformation are investigated using Box-Benhnken central combined design tests and response surface analysis. The response surface model is solved when the temperature rise and stress requirements of the medium frequency transformer are met. The ideal range of insulation material performance parameters is obtained, and the optimized fit scheme is determined combined with the actual: thermal conductivity of 0.8 W·(m·K)−1, thermal expansion coefficient of 4.7×10−5 K−1, and Young's modulus of 3.5 GPa. By optimizing the fit of the parameters of the insulation material, the thermal and mechanical performance of the MF-transformer can be significantly improved. Compared with the pure epoxy resin, the optimized MF-transformer has 82.6 % lower maximum temperature rise, 37.7 % lower maximum stress, and 40.2 % lower maximum deformation, and all index levels meet the requirements. After applying the results of parameter optimization to a prototype MF-transformer, it successfully passed the temperature rise test and thermal shock test, which verified the rationality and reliability of the optimization of the insulation material parameters. © 2023 Chinese Machine Press. All rights reserved.