Research On Modular Multilevel Converter For Traction Drive System

Modular Multilevel Converter(MMC)is a new type of voltage source converter.The advantages of MMC,such as highly modular,easy to expand,low harmonics and high reliability,make it widely used in the field of flexible HVDC transmission.In this dissertation,a single-phase MMC(SP-MMC)which can reduce the volume and weight of traction drive system will be researched.Several key technologies around working principles,topology characteristics,modulation strategy,control strategy,and stability analysis are discussed:The working principle of SP-MMC as a four-quadrant converter is analyzed in detail firstly,and then the mathematical model of SP-MMC is established.Besides,the circulating current characteristics,and the differences among SP-MMC,three-phase MMC and traditional single-phase PWM rectifier are also discussed.What’s more,the secondary ripple on the DC side is focused on,the relationship among the sub-module capacitor voltage,DC voltage and circulating current is researched.Appropriate modulation and control strategies determine whether the converter can operate normally.This paper choose carrier phase shift method as modulation strategy.The control strategies mainly include start-up control,single-phase phase-locked loop,voltage and current dual closed-loop control,capacitor voltage balance control,and circulating current suppression.The simulation and experimental results verify the correctness and feasibility of the used strategies.To analysis the system stability,an impedance model used the harmonic state space theory is studied.In this paper,the open-loop and closed-loop impedance models of the AC side of SP-MMC are established respectively,and the simulation results verified the accuracy of the model.The guiding significance of open-loop and closed-loop impedance models to the selection of main circuit and controller parameters is studied.