| In recent years,high-speed railway plays a progressively important role in the national modern comprehensive transportation.With the development of key equipment technology in China,the demand of the high-speed train of high performance is increasing.Compared with the power-frequency traction transformer in the current high-speed train traction drive system,the train-mounted power electronic transformer(TPET)can minimize weight and volume under the same power,the input-independent-output-parallel(IIOP)DC/DC converter is one of the key equipments in TPET,which influences the volume,weight and power loss.Firstly,the advantages of half-bridge three-level topology over full-bridge two-level topology,half-bridge two-level topology and full-bridge three-level topology are analyzed.The working state,operating principle and corresponding characteristics of the converter under phase-shift control are highly studied,and it is found that there is a large limit condition for realizing soft-switching in all switches of the converter,and there is always a backflow power in the converter,which seriously affects the transmission efficiency.Finally,because the converter has only one control degree of freedom under phase-shift control,it can not optimize both power transmission and reflow power.In order to solve the above problems,a three-degree-freedom DPPS control method is studied in this paper.The Operation State of the converter is deeply analyzed,and the special symmetry relation between the operation modes is explored.Based on the previous research,the DPPS control strategy based on the optimization of the minimum reflux power is proposed on the premise that all switches satisfy the ZVS condition,it is proved that the control strategy has dual relation in the case of k < 1 and k > 1,and the expressions in the range of all values of k are unified.For the IIOP system,which is composed of two-way half-bridge three-level DC/DC converter,the input-output characteristics and structure characteristics of the system are analyzed in depth,and the corresponding state space average model is established.The problems of traditional non-optimal parallel operation control strategy are mentioned,and the control strategy based on reflow power optimization is studied for system efficiency improvement.On the basis of theoretical research,the simulation model of bidirectional half-bridge three-level DC/DC converter and its IIOP system is built by using MATLAB/Simulink,the correctness of DPPS modulation,DPPS control strategy based on minimum reflux power optimization and parallel operation control strategy based on reflux power optimization are verified.The simulation results show that the optimized DPPS control strategy can realize the multi-objective optimization of the reflow power and the effective value of the inductor current under the light load operation,and the minimum optimization of the reflow power under the heavy load operation of the converter,the parallel operation control strategy based on reflow power optimization can attain stable operation in parallel while reducing the system reflow power,and then enhance the system efficiency.Finally,in order to verify the reliability of the research,the low-power experimental platform is built,and the hardware circuit and software control architecture of the platform are designed.On this basis,the experiments of bi-directional half-bridge three-level DC/DC converter and three-module IIOP system are completed.The experimental results further prove that the structure of the main circuit and the control strategy of the system are reasonable. |
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