Research On Three-phase Multilevel Inverter Based On NPC And Switched Capacitor Unit

Owing to the advantages of low harmonic content output voltage waveform,lower switching voltage stress and high energy conversion efficiency,multilevel inverter has gained widespread attention from scholars at home and abroad.When the output level of conventional neutral-point-clamp inverters is increased to five levels,the number of devices used in the inverter will see a dramatic increase.Due to the simple and flexible structure of switched-capacitor unit,it is often used to increase the output level of the inverter,resulting the difficulties of high charging spike current and insufficient energy transfer capability caused by limited capacitor capacity.Therefore,this thesis proposes a hybrid cascaded thirteen-level inverter and a three-phase five-level inverter based with the switched capacitor unit and the traditional NPC inverter structure,which successfully solves the problems caused by the expansion of the output level of the traditional inverter and the difficulty of using the switched capacitors alone in medium and high voltage applications.In addition,the proposed hybrid cascaded inverter structure also successfully solves the current inversion problem in the traditional cascaded H-bridge.Since the three-phase five-level topology is based on the commercial three-level NPC structure with a simple switched capacitor unit embedded,it can speed up the design of the proposed three-phase five-level inverter,thereby accelerating its commercialization process.Finally,the correctness of the proposed inverter topology is verified by theoretical analysis,simulation and experiment.The main works of this thesis are as follows:（1）Introduce the background and significance of the thesis,make a brief discussion on the current development of multilevel inverter and switched-capacitor technology,and introduce the topic of the thesis.（2）To address the problems of current backflow and low output level in the conventional hybrid cascaded H-bridge at certain DC ratios,hybrid three-phase cascaded thirteen-level inverter topology based on three-level neutral point clamping and switched-capacitor unit is proposed.A new hybrid modulation strategy is also proposed to optimize the capacitor voltage ripple and energy loss,and the design of capacitor parameters is given.Finally,the feasibility of the proposed topology and the correctness of the theoretical analysis are verified by simulation and experiment.（3）In order to address the problem of the dramatic increase in clamping devices when increasing the number of output levels of conventional neutral-point-clamped inverter,three-phase multilevel inverter based on nutral-point-clamped inverter and switched capacitor unit is proposed by combining the advantages of the flexibility of switched-capacitor structure.The structural principle,common mode voltage,initial charging of switched-capacitor unit,and suppression of charging current of the proposed topology are analyzed,and the switched-capacitor unit design guiderline is also given.Experiments are conducted to verify the correctness and feasibility of the proposed inverter.（4）Based on the proposed three-phase switched-capacitor five-level inverter,the mathematical model of the grid-connected inverter is analyzed as well as the power double-closed-loop control grid-connected algorithm is designed,and finally the grid-connected operation of the three-phase switched-capacitor five-level inverter is realized.