Characteristics Research And Optimaldesign Of High Power Medium Frequency Transformer

With the development of power electronics device and high frequency magnetic material, power electronic transformer (PET) are increasingly introduced to high voltage high power application field. High power medium frequency transformer(HPMFT) is the key component in the electromagnetic coupling part of PET. It plays the role of electrical isolation and voltage transformation. Because of higher operation frequency, and the windings connection to power electronic converter, the characteristics and optimal design method for HPMFT is quite different from the traditional power frequency transformer. It is of great significance to do a systematic and scientific research for HPMFT when PET is widely used in electric locomotive,offshore wind power and smart grid in the future.This paper emphatically carry out the research on winding loss at high frequency,core loss at high frequency, leakage inductance parameter and optimal design method of HPMFT. The main contents are as follows:AC resistance calculation method for hollow conductor windings in HPMFT is studied. The skin effect and proximity effect for the winding of the transformer under high frequency eddy current are analyzed. The classic Dowell ideal model which used to calculate long-foil type winding loss at high frequency, and the method which other forms of winding converted into long-foil type, are studied. To improve utilization of the winding material and the cooling efficiency of the transformer,hollow conductor is applied to HPMFT. Based on the analysis of numerical results of a large number of finite element models, the relationship between hollow conductor resistance coefficient and relevant parameters and summarized. The application scope of high frequency winding resistance analytical calculation equation is expanded to ac resistance calculation of hollow conductor by the new parameter, hollow conductor resistance coefficient. The experiment verifies the correctness and effectiveness of the proposed method.Several correction methods for core loss calculation under non-sinusoidal excitation are contrastively analyzed. Three kinds of methods for the core loss calculation of the transformer are studied. Several correction methods for core loss calculation based on Original Steinmetz Equation (OSE) under non-sinusoidal excitation are contrastively analyzed by the correction principle. Hysteresis loop measurement system is built, core loss density of a nanocrystalline ring core is measured. The coefficients of OSE are fitted. By core loss density comparision of measured values and analytical calculated values of the correction methods under square and triangle wave voltage excitation for the core, and comparision of finite element simulation values and analytical calculated values of the correction methods under adjustable duty cycle rectangular wave voltage excitation, the correctness of the contrast analysis results in the theory part is confirmed.The calculation and correction method for leakage inductance parameter of HPMFT are studied. The relationships between leakage inductance and leakage magnetic field energy, leakage magnetic flux linkage are analyzed. According to the idea of magnetic field partition, the analytical equation for leakage inductance calculation is derived based on leakage magnetic field energy and leakage magnetic flux linkage. According to the differences between ideal model and actual condition,the correction method is proposed, the conductivity of the winding and the height of the leakage magnetic field are revised. By the comparision of the measured values and analytical calculation values of the leakage inductance for two prototypes demonstrates the correctness of the derived equation, and the error of the analytical calculation can be effectively reduced when the two parameters are revised at the same time. The derivation process and the final equation for the leakage inductance in the winding area can be simplified using an average method when the partial link of the flux in the winding area is neglected. The effectiveness of the method is verified by the comparison of calculated and measured values of a prototype.A 300kW HPMFT in a power electronic traction transformer (PETT) which applied to the high-speed train for lightweight requirement is optimal designed in this paper. Based on system parameters of the PETT and topology and control mode of the power electronic control circuit, design parameters and optimal design process for the HPMFT are confirmed. 24 design groups of core type and shell type structure are putted forward using free parameters scanning method. The winding loss and core loss are calculated for each design group, and the examination of the temperature rise are proceeded. Then, the leakage inductance and weight of the transformer are calculated for each design group. In order to take account to total loss, leakage inductance and mass of the transformer at the same time, comprehensive evaluation coefficient equation is established. The prototype is manufactured according to the optimal solution, and the design target parameters of the prototype are tested. The error between the measured and designed value of the target parameters are all acceptable. It verifies the correctness and effectiveness of the optimal design method.