Design And Research On Electromagnetic Characteristic Of Modular Multilevel High Frequency Transformer

Solid state transformer is an electrostatic device,which combines power electronic conversion technology with high-frequency power conversion technology based on electromagnetic induction principle to realize electricity transform.Through the integration of power electronics technology and traditional transformer,the intelligent level of power grid equipment can be improved.Overall,the solid state transformer topology consists of three parts:the input stage,the isolation stage,and the output stage.The high-frequency transformer in isolation stage requires higher suitability of core materials and core structures.During the magnetization process,the loss caused by the magnetic core material directly affects its transmission efficiency.At present,the calculation method of core loss is not accurate.The thesis studied the design of the modular multilevel high frequency transformer in isolation stage.An improved core loss calculation method was proposed in the core loss calculation process.The main work is as follows:Firstly,the significance of solid state transformer for smart grids and energy internet was discussed.Then the important role of high-frequency transformer in solid state transformer was introduced.In view of the traditional transformer topology,the advantages of modular multi-level high-frequency transformer topology were analyzed.The design methods of geometric parameter(K_g)and area product(A_P)for high-frequency transformer were introduced:By formulating AP method,it provided a theoretical basis for high-frequency transformer design.Secondly,the characteristics of magnetic materials suitable for high-frequency transformer were discussed.The characteristics of high-frequency materials such as iron-based nanocrystal and soft ferrite were compared and analyzed.The characteristics of C-type and E-type magnetic core structures were studied.The specific parameters of C-type and E-type magnetic core structures corresponding to the above two materials were calculated respectively,and four parameters were obtained.The skin effect analysis method was used to select the appropriate wire to find the winding loss.An improved core loss calculation method was proposed to make the core loss calculation more accurate.Finally,the winding equivalent modelling method was adopted to build the COMSOL Multiphysics simulation model respectively for the designed high-frequency transformer according to the above four parameters.And the external circuit joint simulation analysis method was used to analyze and compare the core temperature,loss and electromagnetic characteristic.The results show that the improved core loss calculation method is more accurate.The structure of C-type high-frequency transformer is more efficient than the E-type high-frequency transformer.In the case where the core structures are the same,the high-frequency transformer of the soft ferrite core material is more efficient than the iron-based nanocrystal.