Multiphysics Calculation And Optimal Design Of High-power Medium-Frequency Transformer

With the random distribution of intermittent renewable energy such as solar energy,wind energy and tidal energy,the traditional AC distribution network with unidirectional power flow is rapidly developing to a flexible DC distribution network with multidirectional power flow.Among them,the magnetic coupling DC-DC converter based on the high-power medium-frequency transformer plays an important role in the realization of large-scale DC source interconnection,megawatt-level DC voltage conversion,electrical isolation and power transmission in the DC power grid.However,with the increase of the power density and frequency,the temperature rise and winding deformation of high-power medium-frequency transformers are becoming more and more serious.Therefore,how to accurately obtain the magneto-thermal multi-physics characteristics of high-power medium-frequency transformers and how to achieve fast and efficient performance optimization are extremely important for the development and commercial application of magnetically coupled DC-DC converters.First of all,this article takes into account the influence of temperature on the magnetization and loss characteristics of nanocrystalline alloy materials,and uses the coupled transient electromagnetic-thermal fields method to study the loss and temperature rise characteristics of the medium-frequency transformer under non-sinusoidal excitation.Secondly,the influence of different winding arrangements on the leakage magnetic field and winding electromagnetic force of the medium-frequency transformer is discussed.Finally,the calculation methods for the core loss and winding loss of the medium-frequency high-power three-phase transformer under different winding configurations are proposed,and the overall optimization design method of medium-frequency high-power three-phase transformer is proposed based on the above methods.The specific research contents are as follows:(1)In order to obtain the temperature field of medium-frequency transformer under non-sinusoidal excitation accurately,a measurement platform for the medium-frequency loss and magnetization characteristics of the ring-like nanocrystalline alloy core that takes into account the influence of temperature is built,The core loss and the magnetization characteristics under different temperatures and different magnetic flux densities are measured.The coupled transient electromagnetic-thermal fields method is used to calculate the core loss,winding loss,and temperature field of the nanocrystalline iron core medium-frequency transformer under no-load and short-circuit conditions.By comparing the simulation value with the experimental measurement value,the validity of the calculation method of the coupled transient electromagnetic-thermal field is verified.Based on the above method,the loss and temperature rise distribution of the medium-frequency transformer under the phase shift control mode are calculated.The harmonic loss characteristics and the thermal field distribution law of the medium-frequency transformer winding and iron core under non-sinusoidal excitation are clarified.(2)Firstly,the influence of different winding arrangements on the current density in the conductor area and the leakage magnetic field strength in the core window area are analyzed.Then the three-dimensional finite element models of the medium-frequency transformer corresponding to the different winding arrangements are established.The field-circuit coupling analysis method is adopted to calculate the current density and leakage magnetic field strength of the medium-frequency transformer windings under various winding arrangements.The simulation values are compared with the experimental values to verify the effectiveness of the above model.Finally,the leakage inductance and electromagnetic force of the medium-frequency transformer were extracted by using the current density and leakage field strength of the conductor region calculated by the three-dimensional finite element model.The influence of different winding arrangements on the leakage magnetic field and winding electromagnetic force is analyzed.(3)Based on the analysis of operation principle of isolated three-phase bidirectional dual-active-bridge DC-DC converter(DAB3-IBDC),the influence of Y-Y,Y-△ and △-△winding configurations of medium-frequency three-phase transformer of DC-DC converter on power transmission and soft switching area is studied.Considering the non-sinusoidal voltage and current excitation waveforms under different winding confiurations,an analytical calculation model for the harmonic losses of the windings and iron cores of the medium-frequency three-phase transformer is proposed.A lump parameter thermal network model with 14 temperature nodes is established for the medium-frequency transformer with three-phase five-legs core topology.On this basis,the optimization design process of medium-frequency high-power three-phase transformer based on free parameter scanning method is proposed.According to the optimal design scheme,a 5k Hz/15 k W nanocrystalline core medium-frequency three-phase transformer model witn Y-Y winding configuration is made,and its core loss and winding loss are tested experimentally.The design results are compared with the results of finite element simulation and experimental measurement to verify the effectiveness of the loss calculation method and the effectiveness and correctness of the proposed design method.