Electromagnetic Analysis And Optimal Design Of High Power Medium Frequency Transformer

As power electronic technology has been widely used in various applications including power system,rail traction,wind power generation,electric vehicle and so on,medium and high frequency energy conversion systems,with the core of power electronic equipment and medium and high frequency transformer,become more and more,they reduce the volume and weight and improve the power density by improving the working frequency.In medium frequency energy conversion system,medium frequency transformer is its key part.At present,the applications of medium frequency transformers include converters connected to the power grid,rail traction converters,offshore wind farms,electric vehicle charging piles and so on.This thesis carries out research around the electromagnetic analysis and optimal design of high power medium frequency transformer,in application background of electronic power transformer.The main content includes the following five parts:1)Core loss calculation model for high frequency square wave excitation with ripple is established,and the influences of ripple frequency and amplitude on core loss are analyzed.Winding loss calculation model considering both main harmonic and sideband harmonic is established for the excitation with high content of sideband harmonics.The core loss and winding loss calculation models for high frequency square wave excitation with ripple are used for efficiency optimization of medium frequency transformer.2)Three dimensional finite element simulation model for electromagnetic force calculation of medium frequency transformer is established,and short-circuit electromagnetic forces for different interleaved windings are simulated and compared.Considering different kinds of fault conditions,the maximum values of short-circuit current and inrush current are evaluated.The electromagnetic forces of medium frequency transformer under the maximum short-circuit current and maximum inrush current are simulated and compared,including axial and radial forces varying with the conductor height.The electromagnetic forces under the maximum short-circuit current for two different interleaved windings are simulated and compared,including axial and radial forces varying with the conductor height.The simulation and analysis of electromagnetic force can provide basis for structure design of medium frequency transformer.3)Two dimensional finite element simulation model for electric field of medium frequency transformer is established,and the influence of wire type and winding interleaving on the electric field distribution of medium frequency transformer is analyzed.The electric field distribution,maximum electric field intensity,and electric field distribution in fixed path of medium frequency transformer are simulated.The electric field distributions of windings using different wire types are simulated and compared,and the electric field distributions of interleaved winding and non-interleaved winding are simulated and compared.The simulation and analysis of electric field can provide basis for insulation design of medium frequency transformer.4)An optimization design method of high power medium frequency transformer is proposed,with the aim of minimizing the total loss.First,design considerations are performed from the perspectives of core material choosing,core and wire,insulation design,calculation of leakage inductance,and calculation of temperature rise.Second,the optimization design method of high power medium frequency transformer is proposed,in combination with core loss and winding loss calculation models,and the design flow chart is made.The optimal magnetic flux density is evaluated on the premise of meeting the design requirements,to minimize the total loss of medium frequency transformer,and to optimize the efficiency.5)The silicon steel material with small thickness is chosen as core material,to explore the feasibility of silicon steel material with small thickness in the fields of high power medium frequency application.A 35kW/1.5kV/1kHz silicon steel core medium frequency transformer in electronic power transformer application is prototyped,and experimental research is carried out,including load experiment,no-load experiment,and short-circuit experiment.The contrast experiments are implemented with an amorphous core medium frequency transformer prototype with the same working frequency 1kHz and the similar capacity grade,to evaluate the loss performance,power density and noise level of silicon steel core medium frequency transformer prototype.The results show that,silicon steel material with small thickness has the feasibility in the fields of high power medium frequency application,especially suitable for applications requiring low noise.The experimental research on medium frequency transformer prototype provides reference and experience for the development of subsequent medium frequency transformer prototype.