Research On Wireless Power Transfer System With High Misalignment Tolerance And Efficiency For Electric Vehicles

The industry of Electric Vehicles(EVs)has been included in the "13th Five-Year" national strategic emerging industry development planning.It is faster and more convenient with the technology of wireless power transfer for stationary or running vehicles,reducing the danger caused by charging devices like plug-in cables.Therefore,it can be integrated into intelligent parking technology,realizing intelligent,safe and convenient way of power transfer for EVs.Furtherly,it can provide the basic condition for the technology of battery power feedbacking the grid and peak load shifting,which can improve the quality of the grid.It is essential to develop a wireless transfer system with high misalignment tolerance and reliable energy transfer.Therefore,some researches has been carried out for stationary wireless power transfer systems on several important topics,as loosely coupled transformers,high-frequency compensation networks,system efficiency optimization and control strategies for unidirectional and bidirectional wireless power transfer systems and so on.The research contents are shown as the following four aspects:(1)This paper analyzes and optimizes the structure of loosely coupled transformers and the characteristics of high-frequency compensation networks in wireless power transfer systems.Firstly,based on the finite-element simulated calculation models,the structure parameters of different coupled transformers are optimized to improve the coupling performance of the system transformer,according to the characteristics of the winding and magnetic core arrangement.Furtherly,the shielding performance of passive shielding structures is studied.Meanwhile,the paper proposes a novel transformer shielding structure with guiding cores to improve the coupling performance.Secondly,based on resonant parameters and frequency characteristics,the variations of voltage and current gains and impedance characteristics versus different loads and frequencies are analyzed under tuned and detuned conditions.(2)The paper proposes an efficiency optimization method for wireless power transfer systems with a novel magnetic-circuit model.It is time-consuming for calculating core losses of coupled transformer structures,therefore this paper proposes a novel model magnetic-reluctance model for solenoid transformers,by which the flux distribution of magnetic cores and the around space can be required.Further,core losses of different structure parameters and excitations can be calculated.With systematic loss models,the effect of switching frequency on system efficiency is analyzed.The scale of operating frequency is optimized with a large range.Then the optimal working frequency is determined in the small range under fixed resonant parameters.In addition,the influence of the primary and the secondary turns on system efficiency is further discussed.With the optimized combined range of the primary and secondary turns,the systematic efficiency optimization method can be realized.(3)A unidirectional wireless power transfer system with high misalignment tolerance is proposed.Since misalignment between traditional pads leads to lower coupling and smaller power transfer,the paper proposes a novel coupled transformer structure with two-side single winding for the primary side and single-side double windings for the secondary side.The features of the proposed structure is the compensated couplings performance when there is misalignment along the direction parallelling with the magnetic cores.A series at the primary side and double LCL networks at the secondary side compensation topology is proposed for utilizing the compensated features and achieving better power transfer.With the built equivalent circuit for the system,the influence on the power transfer by the parameters and load variations is analyzed,and the resonant parameters are designed and optimized to achieve the transferred power.(4)The control method for bidirectional wireless power transfer systems is invertigated.Since there is no reliable control without control-level communications for bidirectional systems,a synchronization method for switching phase is proposed by measuring the characteristic of the transfer power at the secondary side.The proposed controller locates at the pickup side of the vehicle.The switching signals from the transmitter side can be derived by measured the high-frequency active and reactive power transfer.The frequencies of the primary and the secondary side can be fixed by a phase control loop.Meanwhile,the phase between the switches can be adjusted for better power transfer when the resonant parameters is detuned.The direction of power flow can be adjusted by the phase control loop,by which bidirectional power transfer can be controlled smoothly.The amount of transferred power can be adjusted by changing the output value of the high-frequency converter at the secondary side.The decoupled performance between the two control loops can be achieved by adjusting switching waveforms.Meanwhile,stable and reliable power transfer can be realized accordingly.