| The disadvantages such as inconvenience and low safety by wired charging restrict the popularization and development of electric vehicles.Magnetic coupled resonances wireless power transfer(WPT)will become the development direction of electric vehicle energy supply by its merits like intelligence,flexibility,environmental protection,low cost and overwhelming advantages when compared with conventional contagious charging.However,The WPT of electric vehicles in motion is a dynamic process,and the fluctuation of parameters may cause the characteristics of the system to change accordingly,making the system deviate from the optimal operating state.The power and efficiency are important indexes to evaluate the performance of WPT system,it is important to optimize these two indexes to improve the performance of the system when parameters change.Power and efficiency are investigated in the thesis.The thesis intents to maintain the system high efficient operation even when conditions change.The influence of parameter changes on system performance is explored by modeling and simulation.Combined with theoretical analysis and experimental verification,the method of optimizing system power and efficiency is studied.The work of the thesis is concentrated on following aspects:The WPT system is modeled.Coupled mode theory,two-port theory and equivalent circuit theory are used to model the WPT system respectively.The application situations,merits and demerits of several modeling methods are compared,and finally the equivalent circuit method is selected.The influence of topology and resonant coil on power and efficiency are discussed.Based on the theory of equivalent circuit,the power and efficiency formulas of SS,SP,PS and PP are derived.The effect of frequency,load resistance and coupling coefficient on system efficiency is discussed by Mathcad 3D simulation.Due to the characteristics such as long distance,medium power and fixed primary compensation capacitance,the SS topology is selected as the research object.In the meanwhile,the influence of coil parameters on system performance is explored from the self-inductance,internal resistance and quality factor of the coupling coil.Coil with small internal resistance and high quality factor is the key to design.The power and efficiency optimization method of WPT system based on impedance matching is proposed.In order to solve the problem of power reduction according to frequency splitting caused by small load resistance value,the system exits the frequency splitting region and improves the output power by adding impedance matching network on the receiving side to make the output impedance greater than the impedance critical value,and the correctness is verified by simulation.The impedance value corresponding to the efficiency extremum is related to the coupling coefficient,so the deviation of the efficiency from the extremum is caused by the fluctuation of the coupling coefficient,the method of maximum efficiency tracking based on impedance matching is proposed to solve this problem.The estimation equation of coupling coefficient is derived,which takes into account the non-resonant state of the system,and the DC side current is adjusted by feedback controller.Impedance matching can be achieved to ensure the maximum transmission efficiency when the equivalent impedance changes by coupling coefficient changing.This method only needs to use DC side data while does not need redundant electrical equipment,more convenient and efficient.Finally,the inverter circuit,coupling coil,rectifier filter circuit and drive circuit are analyzed and designed,and an experimental platform with working frequency of 32 kHz and output power of 200 W is built to verify the effectiveness of the maximum efficiency tracking method based on impedance matching. |
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