Research On Electric Vehicle Charging And Discharging System And Its Control Strategy

With the development of society,environmental pollution and energy problems have become increasingly severe,and the development and utilization of clean energy has become a current research focus.Electric vehicles use clean energy instead of oil to achieve the effect of improving the environment and alleviating energy shortages.Most of the charging piles on the market have only one-way charging function,which cannot make full use of the electric energy of electric vehicles.Therefore,this thesis designs a two-way power converter to dynamically interconnect the power grid and electric vehicles,realize the two-way flow of vehicle network energy,and effectively solve the problem of electric vehicle battery power idle.First,the V2G(Vehicle-to-Grid)technology and various converters commonly used in charging and discharging electric vehicles are analyzed,and a two-stage power converter that can realize charging and discharging functions is designed.Independent design between the two stages,the front-stage bidirectional AC/DC structure is a three-phase voltage full-bridge PWM converter,and the rear-stage bidirectional DC/DC structure is the dual active full-bridge(DAB).The working principles and mathematical models of the front and rear converters are introduced,and the equivalent models and charging methods of commonly used batteries are analyzed to provide a basis for simulation modeling.Secondly,the control system strategy of the charging and discharging system is studied.The front-stage AC-DC conversion adopts the voltage and current double closed-loop control under direct current control,and the voltage outer loop and current inner loop controller are designed.The subsequent DC-DC conversion adopts dual phase-shifting control under the phase-shifting control strategy.In order to improve the service life of the lithium battery of electric vehicles,the DAB converter adopts a constant current and constant voltage control strategy.In the initial stage of charging,the constant current charging controlled by the single closed loop of the current is switched to the constant voltage charging controlled by the double closed loop of voltage and current after the voltage reaches a certain value.In order to meet the needs of grid-connected discharge of electric vehicles,a constant current discharge control strategy of single closed-loop current control is adopted.Finally,a DAB-based electric vehicle charging and discharging system simulation model is built through Matlab/Simulink software,the main parameters of the model are designed,and simulation analysis is performed under different working conditions.The simulation verifies the feasibility of the proposed control strategy.It realizes the first constant current and then constant voltage charging in the charging state,the grid side voltage and current are in the same phase,and the power factor is about 1.The current phase difference is has good dynamic performance;in the charge-discharge switching mode,the control system can well switch back and forth between the charge mode and the discharge mode,with good dynamic response performance and robustness.