Research On Control Technique Of Bi-directional Wireless Power Transfer System For Electric Vehicles

Bidirectional inductive power transfer(BIPT)technique for electric vehicles allows power to exchange between the battery of the electric vehicle and the power grid,and has wide applications.The present studies focus on unidirectional wireless power transfer,and there are fewer studies on bidirectional wireless power transfer.In practical situations,achieving stable and highly efficient bidirectional power transmission is the basic requirement for a BIPT system.To satisfy the requirement,based on the series-series(SS)compensated BIPT system with dual active full bridge(DAFB)structure,this paper has studied the BIPT system in 4 aspects,including the system modelling method,phase synchronization method,power control method and coordinated control strategy.First of all,to analyze the system characteristics and to assist the controller design,the steady-state model of the SS compensated BIPT system with DAFB structure is derived using the fundamental harmonic analysis(FHA)method.The large signal dynamic model of the system is derived using the extended describing function(EDF)method.By linearizing the large signal dynamic model at the operating point,the small signal dynamic model of the system is obtained.The large signal dynamic model and the small signal dynamic model are verified through simulation and experiment.To achieve stable power transmission,the phase of the excitation voltages of the primary side and the secondary side must be kept synchronized.In this paper,a synchronization method with no need of communication,auxiliary winding or complicated hardware circuit,is proposed.In the proposed synchronization method,This method is based on the fundamental phase relationship between voltage and current in steady state.By detecting the fundamental phase of the secondary resonance current,the fundamental phase of the secondary excitation voltage is adjusted in real time to achieve phase synchronization.Based on the dynamic model of the system,considering the coupling effect of the fundamental phase of the secondary excitation voltage and the fundamental phase of the secondary resonance current during the transient process,the proposed phase synchronization method is modeled,and the parameters of the phase synchronization controller are designed based on the model for faster response and better stability.Considering the deviation between the system hardware parameters and the design values,the effects of detuning and mutual inductance coefficient changes on the steady-state performance of the proposed phase synchronization method are analyzed.The experimental results verify that the proposed phase synchronization method has fast dynamic response speed and good steady-state performance,and can still work normally under detuning conditions,and the steady-state characteristics under detuning conditions are consistent with the theoretical derivation.In order to achieve a wide range of power transmission of the bidirectional inductive power transmission system and obtain higher transmission efficiency,this paper proposes a dual-side triple-phase-shift(TPS)control method for the bidirectional inductive power transmission system.This method introduces an additional control variable,which is the fundamental phase difference of the primary and secondary excitation voltages to enable all switching devices to turn on at zero voltage,as well as controlling the pulse width angles to achieve the optimization of the excitation voltage ratio,which reduce the power losses and improve the power transmission efficiency.The experiment results show that the overall efficiency using TPS control is higher than that of dual-phase-shift(DPS)control.In order to achieve stable and coordinated control for the primary and secondary controllers of the bidirectional inductive wireless power transmission system,this paper proposes a dual-side coordinated control strategy based on wireless communication.The coordinated control strategy realizes the data exchange of the primary and secondary side controllers of the bidirectional inductive wireless power transmission system through the lowspeed wireless communication,which can make the system achieve the desired steady-state operating point under the independent control of the primary and secondary side controllers.Taking dual-phase-shift(DPS)control and triple-phase-shift(TPS)control as examples,the implementation of the proposed dual-side coordinated control strategy is introduced,the stability of the proposed dual-side coordinated control strategy is analyzed.The importance of the discrete first-order inertial element on the improvement of the system stability is described.The simulation results show that under the proposed dual-side coordinated control strategy,the system can smoothly reach the desired steady-state operating point.