Research On The Bidirectional Full-Bridge DC-DC Converter For Power Electronic Transformer

Power Electronic Transformer (PET) is an intelligent power device, which combines technology between modern power electronic and power transmission. PET is more than a traditional transformer. It realizes a flexible control of input current and output voltage and has following merits:high density of power, various AC/DC interfaces, wonderful controllability, etc. PET can be widely used among the smart grid and rail traction, but DC-DC transform link is the critical part which achieves the electrical isolation and the power transmission. In this paper, with the isolated bidirectional full-bridge DC-DC converter as the study object, under the method of theoretical analysis, simulation and experimental verification, the DC-DC transform link of PET is studied.Firstly, for the research of the DC-DC transform link that affects the volume, weight of device and the losses of power transmission, the operation principles and characteristics of the DC-DC converter are introduced in this paper. Then, the backflow power and current stress under the control mode of the conventional, unilateral and bilateral phase shifting are modelled and analyzed, and the correctness of analysis is verified by simulations.Secondly, This paper lays emphasis on the study of the two minimum backflow power and current stress control strategies to overcome the disadvantages about the efficiency and the losses of the power transmission. Based on the modulation of the transmission, backflow power and current stress, the realization of the minimum backflow power and current stress are derived. According to the backflow power, the two minimum backflow power-controlled strategies on the basis of single return circuit and soft-switching technology are presented. Similarly, for the current stress, three strategies under the different control modes are studied. Then, the current stress under the three control methods are compared and analyzed. And the simulation results shows the strategies are correct and effective.Thirdly, for the solution of the paralleling of several bidirectional full-bridge DC-DC converters and their current sharing, which exist in the dc link of cascade PET, three control methods are proposed. Based on the disadvantages of the methods, simulations are carried out, and the results demonstrate the validity of theory analysis.Finally, a 3kW experiment platform is designed and built in view of the theory analysis, and the two minimum backflow power-controlled strategies, three new proposed phase-shift-controlled methods, and the current-sharing method are all validated by experiment. Finally, the experiment results prove the validity of theory analysis and simulations, and also show that the control strategies are feasible and effective.