Research On Characterization And Optimization Of Power And Efficiency In Resonant Wireless Power Transfer System

Resonant wireless power transfer(WPT)technology is a frontier science and technic in the field of electrical engineering.Contributed by electromagnetic conversion,the power can be wirelessly transferred,which can effectively solve the problems in the traditional power supply method by power cables,such as wiring aging,point discharge,electric spark and so on.Therefore,the WPT technology can provide a more safety and more intelligent power supply method which can be used in humid environment,rain and snow weather and some other extremity environment.But constrained by the current technical studies,there are still many problems existing in the resonant WPT system,such as the low power transfer efficiency,the short power transmission distance and the small power transmission capacity,which lead to the promotion and application of the WPT technology be seriously limited.There are many technical problems need to be broke through to improve the comprehensive performance of the WPT system.Therefore,in order to effectively improve the performance of the WPT system,the two-coil resonant WPT system with basic Series-Series(SS)compensation topology is selected as the object of this study,and the transmittion power and efficiency of the resonant WPT system are studied in this paper regarding to the core parts of the resonant WPT system.In order to study the wireless power transfer characteristics of the resonant WPT system,the coupling circuit theory is adopted to establish the mathematical model of the whole wireless power transfer system,which includes the coil-system circuit model and the loss model of the power converters used in the WPT system.Based on the mathematical model,the effects on the system power transfer amount,coil-system efficiency and system overall efficiency caused by the five key parameters in the system including DC input voltage,resonant frequency,driving frequency,load resistance and coupling coefficient are detailed analyzed,which provides the theoretical basis and research direction for the research on the improvement of the system peformance.Meanwhile,for better understanding and utilizing the system performance impact factors,three constant output power control method used for variable load systems are presented,and a comprehensive method is proposed by comparing the overall efficiencies of the three methods.With the efficiency optimization method,the key parameters of the system can be optimized matched which can effectively improve the system power transfer efficiency under the premise of realizing constant power output in the variable load WPT system.The correctness of the mathematical model and the constant output power control method are both verified by experiments.Regarding to the problems existing in the SS-WPT system,it is found that increasing the system resonant frequency can effectively improve the coil-system efficiency but significantly limit the system output power.This paper proposes a coil-system parameters design method for different resonant frequencies requirement.With the corresponding coils designed by the presented coil-system parameters design method,when the WPT systems utilize the same DC input voltage to power up the same load,the systems can produce uniform system power level under different resonant frequencies.Meanwhile,in the study of high resonant frequency WPT system,it is found out that with the rapid increase of resonant frequency,the efficiency of the DC-AC inverter in the primary side of the system decreases rapidly due to the increase of switching losses.Therefore,a new method of realizing soft-switching technique by precisely tuning the driving frequency is also proposed in this paper,which can help the DC-AC inverter lossless switch on or switch off under any different resonant frequencies according the system current operation conditions and without changing any hardware in the SS-WPT system.With the proposed soft-switching method,the efficiency of the DC-AC inverter is greatly increased particularly in high frequency systems,and the impacts on the system power level is minimized.The correctness of the design method of coil-system parameters and the soft switching realization strategy are both verified by experiments.Regarding to the widely used flat spiral coils in the current studies of the resonant WPT systems,this paper studies the effects caused by the coil-structure design parameters on the system with limited coil size.The limit size of the flat spiral coils can guarantee the installation limits of the coils in practical equipment.The requirements of improving the power transfer capacity and efficiency are simultaneously considered in this study.Optimized flat spiral coils can be obtained based on the analysis of the impacts on the coil-system physical parameters and the system performance caused by the coil-structure deign parameters.And the optimized coils can greatly improve the comprehensive performance of the WPT system.The correctness of the theoretical analysis in this part is verified by simulation and experiments.Regarding to the compensation topology of the resonant WPT system,the LCC and SS compensation topologies are selected to make a comparison to study the compensation topologies.A new design approach is presented in this paper to show that under certain conditions,the double-sided LCC compensated WPT system can be more energy efficient than an SS compensated WPT system.Subsequently,a DC voltage adjusting method is presented in this paper to make a fair comparison between the SS and LCC WPT systems which can set equal load power for the two systems whilst using identical transmit and receive coils,coil-to-coil distance and load resistance.Based on the LCC-WPT system design approach and the DC input voltage adjusting method,the LCC and SS-WPT systems can produce similar power to the same load with the same coils,and compared to the SS-WPT system,the system efficiency and the ability to resist the coil misalignment of the LCC-WPT system can be greatly improved,the losses of the DC-AC inverter and the voltage stresses on the compensation capacitors of the LCC-WPT system can be greatly reduced.The correctness of the theoretical analysis is verified by simulation and experiments.