Study On Circuit Topology And Its Control Strategy Of Bidirectional Charger Applied To V2G

With the rapid development of electric vehicles and the smart grid, the application of V2G (Vehicle to Grid) has been paid more and more attention. Under these circumstances, the battery pack of electric vehicles should be controlled to interact with grid. Hence, the V2G techniques will be one of the important contents in the smart grid. Among these, the bidirectional on-board charger plays an important role in the energy transfer between grid and electric vehicles. As a consequence, the bidirectional charger topology and its control strategy are mandatory research contents in this thesis.The two-stage circuit topology has been chosen in this paper, in which the single-phase full-bridge PWM topology and the dual active bridge (DAB) topology have been selected to serve as main circuits of bidirectional charger. To improve the efficiency and adapt to high voltage ratio situation, the Parallel-serial DAB (PSDAB) is proposed. The proposed PSDAB circuit is constructed by the paralleled low-voltage terminals of the two DABs and the serial high-voltage terminals of the DABs. And the corresponding drive signals from the two DABs are the same. As a consequence, the phase-shifted control method can be adopted to implement ZVS. The power loss analysis has been,done to compare the proposed circuit to the conventional DAB, and the former efficiency is higher than the latter when in high voltage ratio applications.The control strategy has been studied to achieve bidirectional flows of energy, and the double closed control loop presented to calculate the control parameters. And the control method of building up for dc-bus voltage has been proposed with body diodes rectifying and related modulation techniques given in DAB converter. By applying this method, the phenomenon of inductor saturation can be avoided and hence the current inrush would be cancelled. More importantly, this method does not need additional hardware, and only software can realize. In order to implement the emergency power supply of local load, the seamless switching-shift control between in-grid and off-grid modes is also studied and the digital phase lock loop (PLL) technique presented in this thesis.The parameters of main circuit topology and the control system are designed in this thesis. The design method of resonant inductor is discussed in detail, the design process of transformer given and the power devices selected. Correspondingly, the DSP TMS320F28035is employed to serve as main CPU to implement full digital control for bidirectional charger.The simulations are done by PSIM software and the experiments done by4.6KW experimental prototype, and the simulation and experimental results verify the feasibility and rationality of control strategy and the design of main circuit parameters. And the validity of the proposed PSDAB is suitable for bidirectional charger main circuit to achieve high efficiency when in high voltage ratio applications. Additionally, the PSDAB can realize bidirectional flow of energy, and the seamless switching-shift control between in-grid and off-grid modes. In conclusion, the research results in this thesis can promote the application of V2G in electric vehicle.