| Wireless power transfer(WPT)technology can realize no-catenary power supply,and has the advantages of safe operation,convenience and flexibility,providing a new technical scheme for modern tram power supply.In order to solve the problem of charging speed and operating range of trams without overhead line system,the hybrid energy storage system composed of super-capacitor and batteries has attracted more and more attention.The LCC-S WPT system,which is suitable for high-power scenarios,has high harmonic content and serious distortion in the output current of the inverter under light load,resulting in large inverter loss and low system efficiency.This is particularly common under load conditions with large power variations such as super-capacitor.In this paper,the inverter current distortion of LCC-S wireless charging system is studied,and the corresponding solution is proposed.This paper first analyzes the basic structure of the tram wireless power transfer system,including power converter topology,control mode and equivalent analysis method.The common coupling coil and basic resonance compensation structure are introduced.The characteristics of LCC compensation structure and the design method of resonance parameters are summarized,the constant voltage characteristics of LCC compensation structure under variable load are analyzed,and the advantages of LCC compensation structure in the wireless charging application of tram are explained.Then this paper takes the LCC-S WPT system as the object,analyzes the formation mechanism of the output current distortion of the inverter and makes a quantitative analysis of the harmonic content of the output current.Based on the derived LCC impedance expressions,the characteristics of fundamental impedance and high order impedance under different load conditions are analyzed.The reason of output current distortion of the inverter under light load conditions is explained.Then the harmonic content and the current value at switching time are quantitatively analyzed by the time domain expression of inverter current.On this basis,the influence of the inverter current distortion on the WPT system is analyzed.The switching loss of the inverter is increased due to the large current value at switching time of the inverter.As a high frequency system,the system efficiency under light load condition is greatly reduced.The third,this paper presents a method to suppress the distortion of inverter current from the perspective of compensation structure,and proposes an improved LCC compensation structure.The improved LCC compensation structure still has constant voltage output characteristics and does not change the basic transmission characteristics of WPT system.Considering the resonant element stress,the frequency bifurcation characteristics between the origin and improved LCC compensation are analyzed by impedance analysis,and the parameter design method of the improved LCC compensation structure is given.Simulation results show that the proposed method is effective to suppress the inverter current distortion.Finally,the correctness of the theoretical analysis was verified by a low power experimental platform.The 30 k W wireless power transfer experiment platform was built.The construction process of the experimental platform is introduced in detail.The preliminary debugging work of the platform was summarized,and the low-voltage functional experiment was completed. |
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