Research On Optimization Control Strategy Of The Power Electronic Traction Transformer

Power electronic transformer is a new type of transformer which combines the power electronic conversion technology with the traditional core frequency transformer to realize voltage transformation and energy transfer. It has the advantages of small volume and weight, high reliability, high power density, etc. Power electronic transformer used in the locomotive traction system, which can make the traction transformer realize miniaturization and lightweight, has great significance for traction drive system to develop in the direction of high speed and high power. This paper mainly studies the application of power electronic transformer in the locomotive traction system and focuses on the control strategy for each link of the power electronic traction transformer to optimize the performance of its efficiency and power quality.In the first stance, the overall structure of power electronic traction transformer and its working principle are analyzed. In order to improve its rated power and voltage level, power electronic traction transformer is composed of multiple unit modules. And each PETT unit module is composed of a single phase AC/DC single-phase rectifier and a bidirectional DC-DC converter. This paper established the mathematical model of power electronic traction transformer unit, which including the input stage model and the isolated stage model and the input series output parallel cascade topology model of the power electronic traction transformer.Secondly, based on the constructing mathematical model of the power electronic traction transformer unit module, the control strategy of the single phase AC/DC converter and DC-DC converter in the unit module is studied. Among them, according to the mathematical model of single-phase PWM rectifier, the reason which produces the low order harmonics in AC side is analyzed, pointing out the disadvantages of traditional PI control, and based on the quasi PR control theory and the harmonic compensation technology, a new control strategy is introduced for the single phase PWM rectifier of electric locomotive traction system. Compared with the PI control, this control strategy can effectively suppress the harmonic of the grid side current and improve the transient performance of the system. In the DC link, the soft-switch and the backflow power characteristics as well as the relationship between them under the dual phase shift control are analyzed for bidirectional DC-DC converter. On this basis, this paper proposes a dual-phase-shifting optimal control algorithm, the control algorithm regard the reduction of the backflow power and expand the scope of "soft-switching" of the switches as the constraint condition. Through the optimal selective of the phase-shifting angle, coordinating the requirement of the "soft-switching" scope and the backflow power, making the whole switch tubes realize the "soft-switching" and at the same time restrain the backflow power effectively, so as to improve the efficiency of the converter.In addition, on the basis of the above research, the experimental prototype of a power electronic traction transformer is built. The control system of the prototype is combined DSP with FPGA. The main circuit parameter of PETT are designed, The design method of the filter in the link of the rectification and the high frequency is analyzed emphatically. The proposed control strategy is further verified by the prototype experiment. The experimental results show that the proposed control strategy can effectively improve the power quality of the power electronic traction transformer and improve the efficiency of the system.Finally, based on the topology model of the input series output parallel of cascading power electronic traction transformer, the reasons for the imbalance between the input voltage and the output current and the relationship between them are analyzed. The research result points out that the input voltage balance can not ensure the output current balance, and then a control strategy based on the front and back stage coordination control to realize the input voltage and output current sharing is proposed. On the basis of single phase dq vector control, the input voltage sharing control is proposed on the input stage, and based on the principle of the phase-shifted control, the output current sharing control is introduced on the isolated stage. This controller takes the actual measured value of output dc current as the feedback signal, which has no complex power calculation and is easier to be implemented. Simulation results show that the proposed control strategy achieves the input voltage sharing and the output current sharing.