Research On Temperature Rise Characteristics Of High Frequency Transformer Based On Multi-physical Simulations And Thermal Circuit Model

With the rapid development of new power system,high frequency transformer is widely used because of its small size,light weight,high efficiency and power density.The high-frequency characteristics cause the loss of the core and winding of the high-frequency transformer to increase sharply,and the large loss causes the temperature to rise rapidly,affect the safety and stability of the working environment.Therefore,it is of great significance to study the electric,magnetic,current and thermal characteristics of high frequency transformer and the influence of different working conditions on its temperature rise characteristics.Around the above questions,the main research content of this paper is as follows:Firstly,the mathematical and physical models are established to study the electromagnetic loss characteristics of high frequency transformer.Based on the separation theory of core loss,considering the actual operating conditions of high frequency transformer under non-sinusoidal excitation,the eddy current loss and residual loss under different duty cycle and trapezoidal wave excitation with different lift time coefficients are derived,and their coefficients are further modified to include magnetic induction variables,the calculation precision under high frequency operation is improved.Using the finite element software Maxwell to model the high frequency transformer,the nanocrystalline is selected as the core material,and the effect of frequency and winding arrangement on the distribution of electromagnetic field and loss is studied.Secondly,the loss generated in the electromagnetic field simulation is used as the heat source,which is introduced into the flow heat simulation field by Workbench platform,a multi-physical field coupling model of electric field,magnetic field,flow field and thermal field is established to investigate the effects of different flow angles and fluid velocities on steady temperature rise and thermal region distribution of high frequency transformer.The results show that the hot spot temperature is affected by the angle of flow,and the steady temperature can be reduced by the increase of flow velocity.Finally,based on the core structure and winding layout of core-type high-frequency transformer,the distribution of cooling air in different regions is considered,a five-node transient thermal circuit model for core-type high-frequency transformer considering nonlinear thermal resistance is proposed,and the calculation accuracy of different fluid velocity,different load coefficient and different ambient temperature under constant load is verified,and the calculation precision under the impact load and time-varying load,based on which the Nusselt number is modified.The results show that the average steady-state error is about 5.75% under steady-state load,and the maximum error is 8.61% under steady-state load,shock load and time-varying load,using the modified Nusser number to calculate the convective thermal resistance can reduce the percentage of temperature error by about 3%,which provides a simple and low-cost calculation method for predicting the hot spot temperature of high-frequency transformer in engineering practice.