| With the continuous integration of DC loads such as large-scale clean energy grid-connected power generation,electric vehicles and high-speed railway traction locomotives into the power system,new challenges have been posed for the planning,management,and safe and economic operation of the power system.Energy conversion system is an important part of connecting power grid and DC load,which needs stability and efficiency.However,with the increase of power density and frequency,iron-core solid-state transformers based on power electronics technology will have problems such as increased loss,excessive temperature rise and accelerated insulation aging.The large-capacity high-frequency air-core transformer realized by magnetic resonance technology has the advantages of lower manufacturing cost and no core loss,and has broad application prospects in power systems.However,large-capacity high-frequency magnetic resonance air-core transformers also have disadvantages such as low transmission efficiency,high excitation current,and less stability than iron-core transformers.The study of electromagnetic parameter characteristics and optimization design method of large-capacity high-frequency magnetic resonance air-core transformer will help to provide reference and help for the practical application of the transformer.Firstly,considering the influence of twisted structure and pitch on the current distribution of strands of Litz wire,this paper proposes a high-frequency loss analysis method of non-perfect twisted Litz wire by combining the segmented equivalent circuit method and finite element method.Secondly,a multi-objective optimization design method of high-frequency magnetic-resonance air-core transformer is proposed based on parametric scanning method.Finally,according to the design results of the air-core transformer,the temperature rise characteristics of the air-core transformer are analyzed by electromagnetic-thermal-fluid coupling simulation,and the influence of different wind speeds on the heat dissipation effect of the air-core transformer is clarified.The specific research contents are as follows:(1)The precise mathematical characterization of high frequency loss of non-perfect twisted Litz wire is studied.Firstly,the magnetic field distribution,current density distribution and twisted structure characteristics of the Litz wire under high frequency conditions are systematically analyzed.Then,based on the twisted structure of the Litz wire,the accurate calculation method of the loss of the Litz wire is established by the segmented equivalent circuit method and the finite element method,that is,the Litz wire within the complete twisted pitch is equivalent to the inductance cell of the linear structure after being segmented,and the impedance matrix between the strands is calculated by the two-dimensional finite element method,and then the strand current vector is solved according to the system equation.Finally,the new method,the traditional analytical method and the three-dimensional finite element method are compared and analyzed from the perspective of calculation accuracy and quantity for three typical Litz wire twisted structures.The results show that the new method can quickly and accurately calculate the high frequency loss resistance of Litz wire with different twisted structures,and the maximum relative deviation is less than 2.34%.The influence of twisted structure and frequency on the high frequency loss resistance and current flow mode of Litz wire is clarified by the new method.(2)The optimization design of high-frequency magnetic-resonance air-core transformer is studied.Firstly,considering the influence of winding structure and arrangement on winding self-inductance and coupling coefficient,two groups of multi-layer coaxial cylindrical winding structure and interleaved winding technology are proposed to improve the inductance and coupling coefficient of the primary and secondary sides.Secondly,considering that the main magnetic circuit of the air-core transformer is made of non-ferromagnetic materials,the spatial magnetic field distribution is relatively complex,and the self-inductance,leakage inductance and coupling coefficient of the primary and secondary sides are difficult to be accurately obtained by analytical methods.Based on the research of the calculation method of the high frequency loss of the Litz wire,the magnetic shielding design method,etc.,the multi-objective optimization design process of the high-frequency magnetic-resonance air-core transformer is established by using the parametric scanning method and the orthogonal experimental method.Finally,a 81.34k Hz/2k V/100k W air-core transformer model is designed according to the above method.The efficiency,weight power density and volume power density are 99.61%,21.6k W/kg and 5.1 k W/dm~3,respectively.(3)The temperature field of high-frequency magnetic-resonance air-core transformer designed in this paper is studied.An electromagnetic-thermal-fluid coupling simulation platform was established to analyze the temperature rise of the air-core transformer under forced air cooling conditions.The flow field distribution and the convective heat transfer coefficient of the fluid-solid coupling surface were used to prove the correctness of the coupling model and the temperature field analysis results.Finally,the influence of different forced convection conditions on the heat dissipation efficiency of the air core transformer is analyzed by adjusting the air outlet wind speed,which has certain reference significance for improving the economic operation capacity of the air core transformer. |
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