Research On Bidirectional Wireless Charging Systems Of Electric Vehicle

With the increasing number of electric vehicles,the charging problem of electric vehicles has become the focus of current research.As a safer charging method,wireless energy transmission technology provides new possibilities for charging electric vehicles.The bidirectional wireless power transfer(BD-WPT)system can support bidirectional flow of electrical energy.Electric vehicle clusters with a certain scale can be used as distributed energy sources to alleviate the pressure on the grid.Hybrid energy storage system(HESS)combines the advantages of high energy density and high power density,and is used to improve the performance of electric vehicles.In order to take advantage of BD-WPT system and HESS,this thesis focuses on the BD-WPT system based on HESS.The main research work and innovations of this thesis are as follows:(1)Design of system compensation structure.Mathematical derivation of the four compensation structures S-S,S-LCC,LCC-S,LCC-LCC,comparing the transmission power,transmission efficiency and voltage transmission capability of the four compensation networks,finally takes the LCC-S compensation structure with constant voltage output characteristics.(2)Design of control strategy for system charge and discharge.Aiming at the LCC-S compensation structure,a battery constant voltage and constant current charging control strategy based on the BD-WPT system is designed.By adopting a phase-shifting control strategy,the system can ensure constant voltage and constant current mode switching when the energy flows bidirectionally.(3)Design of system power allocation strategy.The power distribution strategy of BD-WPT system for HESS is designed.In the wireless charging and discharging process of electric vehicles,by designing the power distribution strategy of HESS in six working modes,the super capacitor is responsible for high-frequency power fluctuations.When the required power is greater than the battery power,the super capacitor assumes the remaining power in a short time,improving the charging and discharging conditions of the battery.(4)Design of automatic alignment control strategy for system transmission coil.In order to improve the transmission efficiency of energy,it is necessary to ensure that the transmission coil is in the aligned state.The transmission coil automatic alignment control strategy is designed.When the transmission coil is offset,increase the output power of the primary side to ensure that the output power of the secondary side is constant.When the current on the power supply side is the smallest,the transmission efficiency of the system is the largest,and the transmission coil is in the aligned state.Therefore,by detecting the current on the power supply side in real time and controlling the motor to drive the transmitting coil for position search,the efficiency of energy transmission is improved.(5)The simulation system and experimental platform are built,and the proposed control strategies are verified by simulation system and experimental platform.The results show that the research method and control strategy proposed in this thesis are feasible.