Electromagnetic Characteristics And Optimal Design Method Of High-voltage High-Frequency Transformer With Large Capacity

The high-power DC-DC converter is the key equipment to establish and develop the large-scaled DC source interconnection,the MW-level DC voltage conversion,and the DC grid.The magnetically coupled DC-DC converters based on high-frequency transformers(HFTs)have obvious advantages on realization of the galvanic isoltaion between primary and secondary sides and the voltage conversion.Therefore,improving the power capacity,the efficiency and the power density of a single high-frequency transformer,and reducing its size and weight are extremely important for the development and commercial applications of high-power DC-DC converters.The main work of this paper includes:(1)The expressions of several modified Steinmetz equations are extended to determinate the core losses under square and trapezoidal voltage excitations.The experimental platform which can be used to test the high-frequency loss of magnetic materials is built,and the core losses of several ring cores excited by sinusoidal and non-sinusoidal voltage waves are measured.The loss properties of different magnetic materials under different flux densities,frequencies and excitations are compared.By comparing the measured values and analytical calculated values of core losses under adjustable duty cycle square wave and adjustable rise time coefficient trapezoidal wave,the correctness of the theoretical analysis result is verified.Based on statistical loss theroy(STL),an improved loss separation model is developed to extract the eddy-current,hysteresis and excess loss components in a wide range of flux density and frequency.Some important factors,for example,skin effect,complex relative permeability,nonlinear relation of B(H)and non-uniform flux density profile are considered.The calculated losses in an ultra-thin grain-oriented(GO)steel with 0.08 mm thickness are compared at various frequencies and peak flux densities with the measured losses from a standard single sheet tester,and the correctness of the improved loss separation model is confirmed.(2)A semiempirical calculation method for the AC resistance factor considering the edge effect is proposed for the accurate determination of HF copper losses in high-power HFTs with the layered windings.The horizontal component of magnetic field located just on the top of the winding edge is analyzed.The calculation accuracy of Dowell’s equation and modified Ferreira’s formula at different porosity factors are compared.All the determinant geometrical variables that govern the AC resistance factor or copper losses are selected by using the sensitivity analysis,and are compounded to create several nondimensional generic parameters,which are then gathtered into the semiempirical formula.The correction coefficients are fitted by the multivariable regression analysis based on the intensive simulation results obtained from the parametric 2-D finite-element method(FEM)simulation model of the HFT.The effectiveness of the method is verified by the comparison of calculated and measured values of a HFT model.The impact of porosity factor and interleaving winding configurations on the AC resistance factor of winding is investigated.(3)Based on 2-D FEM analysis,the relationship between leakage magnetic field,winding conductor current density and leakage inductance are analyzed,and the effect of different winding configurations on electromagnetic parameter of HFT is studied.Expressions of magnetic field intensity in insulating layer and copper foil conductor are derived based on Ampere loop theorem and 1-D diffusion equation,respectively.According to the idea of magnetic field partition,the analytical equation for leakage inductance calculation is derived based on leakage magnetic field energy.The analytical method is derived for the winding with a single foil per layer(i.e.,nonporous layer)and is extended to the porous conductor layer(having many turns in each layer)by using the porosity factor.The impact of interleaving winding configurations on the leakage inductance in a wide range of frequency is investigated.The effectiveness of the method is verified by the comparison of calculated and measured values of a HFT model.(4)Based on the analysis of operation principle and equivalent circuit model of the isolated bidirectional dual-active-bridge DC-DC converter(DAB-IBDC)in power electronic transformer(PET),an analytical calculation method for harmonic current in HFT is proposed.On the basis of core loss,copper loss,leakage inductance and temperature models,the design methodology is established by using the free parameters scanning method.Many candidate solutions are enumerated and tested one by one to determine the solution set that satisfies various constraints.Two models of 5kHz/10kW nanocrystalline-based HFTs with shell and core type topologies are designed and manufactured.Extensive measurements and FEM simulations are performed on the constructed models in order to extract the leakage inductance,copper loss,core loss,and temperature rise.The correctness and effectiveness of the design methodology are proved.(5)Based on system parameters of PET,and topology and control strategy of the power electronic control circuit,a novel computer-aided optimal design method of HFT based on a multiobjective non-dominated sorting genetic algorithm Ⅱ(NSGA-Ⅱ)is proposed.The optimization methodology has the aim of reaching the maximum power density and the maximum efficiency,and also,transformer leakage inductance at the same.Transformer operating magnetic flux density is taken as design variable,while maximum allowable temperature rise,insulation level and load loss are taken as constraints.The proposed methodology and the optimal solutions are validated with the design of a 200kVA/10kHz HFT prototype used in a PET which applied to the flexible substation.The prototype is manufactured,and some simulation and experimental results are presented so as to demonstrate the proposed models and the performance of the built transformer.The error between the measured and designed value of the target parameters are all acceptable.It verifies the correctness and effectiveness of the optimal design method.The resulting optimal design of 200kVA/10kHz HFT prototype with nanocrystalline cores and square litz wire can achieve an efficiency of 99.45%,a power density of 8 MW/m3,and a forced air-cooling temperature of 62℃.