Multi-objective Design And Insulation Structure Optimization Of Multi-winding Medium-frequency Transformer

With intermittent renewable energy connected to the grid,the distribution network is developing rapidly to a flexible DC distribution network with multidirectional power flow.Among them,the magnetic coupling DC-DC converter based on high-power medium frequency transformer plays an important role in realizing large-scale DC source interconnection,megawatt-level DC voltage conversion,energy transmission and electrical isolation in DC power grid.However,the temperature rise and strong local electrical stress of high frequency transformer have becoming important factors limiting the development of large capacity and medium frequency transformer.Therefore,it is crucial for the development and application of the magnetically coupled DC-DC converter to obtain the design parameters of medium frequency transformer accurately,optimize the design quickly and effectively,analyze the electromagnetic thermal multi-physical field characteristics of the medium frequency transformer and consider the electrical stress to ensure the reliability.Firstly,based on the loss calculation,it obtains the maximum capacity of the medium frequency transformer at a certain frequency through analytical calculation,and then derives the optimal magnetic flux density,and calculates and analyzes the transformer design example.Secondly,the calculation method of core loss and winding loss of medium frequency multi-winding transformer is proposed,and the overall optimization method of multi-winding medium frequency transformer is proposed based on the above method.Finally,according to the research of the above problems,in order to fully consider the electrical stress margin of the multi-winding medium frequency transformer,an optimization method of transformer insulation structure based on the response surface method is proposed,which provides guidance and basis for the structural design and insulation design of the medium frequency transformer.The specific studies as follows:（1）Aiming at the parameter design problem of medium frequency transformer.Firstly,based on the calculation method of transformer loss,the maximum design capacity of medium frequency transformer and the relationship between core magnetic density and working frequency are obtained.Secondly,the expressions of frequency,design size and power density of medium frequency transformer are derived,and the effects of amorphous,nanocrystalline and ferrite of different ferromagnetic materials on power density are analyzed.Finally,the calculation method of the optimal magnetic density of the core is analyzed,and the transformer loss under the optimal magnetic density is obtained,and the efficiency optimization of the transformer is realized.The effectiveness of the method is verified by an example,which lays a foundation for the optimal design of the medium frequency transformer.（2）Aiming at the optimization design problem of multi-winding medium frequency transformer.Firstly,combined with the working mode of multi-winding medium frequency transformer,the calculation formula of core loss,winding loss,optimal diameter and number of strand of Litz wire are given.Then taking the isolation distance between the primary and secondary windings,the isolation distance between different primary windings,and the isolation distance between different secondary windings as the factors affecting the medium frequency transformer,the 3-factor and 5-level orthogonal experiment plan is obtained from L₂₅（5³）orthogonal experiment table.All designations are simulated by Ansys platform.The influence law of the two evaluation indexes of leakage inductance and loss is explored.The optimal combination scheme is determined by the combination of visual analysis and range analysis.Finally,the optimal scheme of a nanocrystalline core multi-winding medium frequency transformer with rated parameters of 10kHz and 50 kW is presented,which verifies the effectiveness of the design scheme.（3）Optimal design of insulation structure for multi-winding medium frequency transformer.The method of adding electrostatic ring and angle ring is proposed to suppress the electric field strength at the end of primary winding,and the electric field strength distribution of multi-winding transformer is calculated by electrostatic field finite element analysis method.Combined with the sensitivity analysis method,the influence of the size parameters of the electrostatic ring and the angle ring on the electric field strength is studied,and the decisive factors of the maximum electric field strength are determined.The response surface function related to electric field strength is constructed by response surface method to optimize the insulation structure of transformer,which provides guidance for the electrical stress softening of medium frequency transformer.