Design And Study For Nano-Crystalline-Based High-Frequency Transformer In Dual Active Bridge Converters

With the distributed power supply and load connected to the DC distribution grid,high-power power electronic converters which can potentially replace traditional low-frequency transformers have become the research hotspot.In recent years,one of the most cited terms of these converters is the solid state transformers(SST),which is a new transformer technology that combines power electronic conversion technology with high frequency power conversion technology.High-frequency power transformers are considered as the key element of SSTs The main requirements of SSTs,i.e.,high power density,high efficiency,voltage adaptation and isolation requirements are to a great extent fulfilled through a careful design of high-frequency power transformers.Based on the bidirectional dual-active-bridge(DAB)isolated DC-DC converter topology,this paper studied the nanocrystalline core material and proposed a design and optimization method for the high-frequency transformer.The design of the nanocrystalline-based high-frequency transformer was completed and the efficiency of the DAB converter was evaluated.The main work of this paper is as follows:The different calculation methods of core loss were summarized,and the Steinmetz equation-based core loss models under non-sinusoidal excitation were modified.With the finite element simulation model established in ANSYS MAXWELL,the accuracy and applicability of the core loss models under high frequency conditions were verified.A new design and optimization method for high frequency transformers was proposed.According to this design method,the optimization process of free parameter scanning was carried out,and the Pareto frontier curve of power density-efficiency was drawn.Finally,utilizing the proposed design method,a high-efficiency,high-power density nanocrystalline-based high-frequency transformer was designed.The loss of DAB converter with the nanocrystalline-based high frequency transformer was analyzed.The theoretical loss of DAB under load condition was calculated.The experimental prototype was built and tested.The results showed that the efficiency of the converter increases first and then decreases with the increase of load power,and it can achieve an efficiency of 97.2% at 1.5kW.Then the comprehensive optimization problem of DAB converter with frequency varying is analyzed.The DC bias phenomenon of the DAB converter was systematically studied.By applying the DC component to the primary side current of the high-frequency transformer,the phenomenon of DC bias was studied.The influence on the loss of transformer was analyzed.A new method was proposed to measure the dc bias,the results showed that the measuring method could work effectively.A separate proportional integral(PI)control was designed to eliminate the influence of DC bias.The dynamic response and controlling effect for dc bias were all verified in experiments.