Research On Control Strategy Of Full Bridge Three-Level Bidirectional DC-DC Converter

Power electronic transformer has become a hotspot in global research.The light weighting of high-speed railway is a developing trend.The replacement of traction transformer and four-quadrant rectifier in traditional traction transmission system by power electronic traction transformer(PETT)is inevitable in the future.As the core component of PETT,bidirectional DC-DC converter is one of the core component of PETT,its power density,energy consumption and stable operation state are of great concern to the performance of PETT.The full bridge three-level bidirectional DC-DC converter studied in this thesis has the advantages of high voltage resistance and large current resistance,and it is more suitable for engineering implementation in high-power applications.Therefore it is of great practical significance to study its high-efficiency control strategy.In this thesis,the full bridge three-level bidirectional DC-DC converter is studied.First of all,the structural advantages of full bridge three-level bidirectional DC-DC converter over full bridge two-level,half bridge two-level and half bridge three-level are compared and analyzed.The working principle,transmission power characteristics,soft switching characteristics and reflux power characteristics of the full bridge three-level bidirectional DC-DC converter under the phase shift control strategy are analyzed in detail.On this basis,it is found that the full bridge three-level bidirectional DC-DC converter under phase shift control cannot realize soft switching in the full power range,and there is a problem of large reflux power.In order to solve the above problems,a single PWM plus phase shift(SPWMPS)control method with two control degrees of freedom is proposed for full bridge three-level bidirectional DC-DC converter.The working principle and working characteristics of the converter under the SPWMPS control method are analyzed in detail.It is found that the control method can effectively increase the soft switching range of the converter.Aiming at reducing the reflux power of the converter,a minimum reflux power optimization control strategy is proposed,which is called optimized SPWMPS control.The control strategy enables the converter to achieve soft switching in the full power range,which can effectively reduce the system reflux power.The simulation results verify the correctness of the theoretical analysis.However,under this control strategy,the converter has a large load current stress and a low efficiency during light load operation.Furthermore,in order to further improve the efficiency,a dual PWM plus phase shift(DPWMPS)control method with three control degrees of freedom is proposed.The working principle and working characteristics of the converter under the DPWMPS control method are analyzed in detail.Aiming at reducing the reflux power of the converter,a minimum reflux power optimization control strategy is proposed,which is called optimized DPWMPS control.Theoretical analysis shows that the optimized DPWMPS control strategy can effectively reduce the current stress at light load and optimize the light load efficiency.The simulation results verify the correctness of the theoretical analysis.Last but not least,a low-power experimental platform for full bridge three-level bidirectional DC-DC converter is built,and the dynamic response of the system is verified.Then,the working modes under three control methods of phase shift,optimized SPWMPS and optimized DPWMPS are verified,and the reflux power in three cases is analyzed.The experimental results verify the feasibility and effectiveness of the proposed modulation method and two optimized control methods.The experimental results show that the two optimal control methods are effective in reducing the reflux power of the converter,and the optimized DPWMPS control can effectively reduce the current stress at light loads.Besides,the efficiency of the converter under the three control strategies at different power rate is measured and compared.The efficiency curve shows that the optimized DPWMPS control strategy can effectively improve the efficiency of the full bridge three-level bidirectional DC-DC converter compared with the phase shift control strategy and the optimized SPWMPS control strategy.