Research On Power Capacity Enhancement Of Wireless Power Transfer Systems Based On Combined Inverters

Wireless Power Transfer(WPT)technology is an emerging technology,which can transfer power via the air gap to electronic loads without physical contact.The technology is a revolutionary breakthrough compared with the way of traditional contact power supply,which can effectively address a series of problems caused by the traditional contact power supply(such as the mechanical sliding friction problems caused by catenary and the third track in railway transportation).Compared with the traditional contact power transmission technology,such systems have numerous advantages,e.g.,safety,reliability,flexibility and other advantages.However,in high power applications such as railway application,the traditional WPT system may hardly meet the demand of high power transmission due to the limitation of the capacity and cost of the switching devices which construct the high frequency inverter.Moreover,due to the drawback of the single energy transmission channel and the relatively large coil volulme,it is difficult to achieve high power energy transmission targets considering high safety,high flexibility and high reliability of WPT systems.Therefore,this thesis aims to improve the capacity of the WPT system by using cascaded multi-level inverter,parallel inverter and multi-coil energy transmission technology to enhance energy transmission level.This thesis focuses on solving the problems caused by the application of the aforementioned power enhancement technologies.The main work of the thesis can be summarized as follows:(1)In order to solve the problem of high voltage stress in switching devices,a WPT system based on cascaded multi-level inverters is proposed.Taking the five-level inverter with two cascaded H-bridges as an example,the working principle and auto power-balance characteristic are analyzed in detail.According to the harmonic phasor of the output voltage and the harmonic elimination equation,the output voltage of the specific harmonic can be eliminated by selecting the appropriate switching angle.Finally,the simulation and experiment are carried out to verify that by applying the proposed method,which can effectively eliminate the specific harmonic componcents(eliminating the third harmonic as an example),the output power of each cascaded inverter is identical,and the system output power can be regulated continuously.(2)In order to address the issue of high current stress in switching devices,a WPT system based on multiple paralleled inverters is proposed.The influence of the tolerance of the devices and voltage on the circulation between the parallel units is analyzed.To address the problem on the angle detection method with high frequency currents,a virtual active and reactive power decomposition method without phase locked loop which reduces the complexity of control algorithm is proposed.Finally,the experimental results show that the proposed method can effectively eliminate the circulating current among the paralleled inverters.The protection devices are added in the system,which can remove the faulty unit in the soft switching and increase the system stability and reliability.(3)In order to resolve the problem that the traditional WPT system energy transmission channel is single,a WPT system based on multiple transmitter coils is proposed.The use of multiple transmitter coils increases the number of system energy transmission chhannels,while effectively reducing the coil volume,the number of turns,and the core weight.Considering the influence of crossing coupling mutual inductance of the multiple transmitter coils,the capacitance compensation method is proposed,which can not only make the inverters work in the resonant condition,but also can effectively distribute the output power of each inverter.Meanwhile,an optimal load tracking method is proposed,which realizes the track of the optimal efficiency point of the WPT system in real time and makes the WPT system maintain high transmission efficiency in high power transmission.(4)Based on the proposed multiple transmitter coils WPT system,considering the tolerance of the resonant components and the voltage,the influence of the resonant components' tolerance,the current of each transmitting coil is analyzed.A coil current sharing control method is also proposed.The theoretical and experimental results verify the proposed control method,and the system's transmission power,efficiency and reliability are significantly improved.(5)In the application of rail transit,the receiver coil's size is limited by the installation space at the bottom of the vehicle.A novel WPT topology which consists of dual overlapped transmitters and dual receivers is proposed.A three-dimensional finite element analysis tool is adopted to investigate the proposed magnetic coupling structure and decoupling transformers.Besides,the effects of the crossing coupling mutual inductances among the transmitters and receivers are analyzed.Decoupling transformers are employed to mitigate the effects of the same-side cross couplings.Finally,the experiment validates the effectiveness of the proposed method.The proposed method not only improves the overall efficiency of the system,but also can reduce the reactive power of the inverter and provide system redundancy.This paper has formed a step-by-step power enhancement strategy,from "multiple power supplies,single transmitting coil and single receiving coil" to "multiple power supplies,multiple transmitting coils and single receiving coil",to "multiple power,multiple receiving coils and multiple receiving coils".The problems of hannonic,current circulation,power imbalance,coil volume and reliability are well discussed and solved in the paper.The above-mentioned power enhancement technologies provide theoretical basis and experimental basis for the application of the above-mentioned power enhancement technology in high power WPT system.