Research On Control Strategies And Design Of MMC In VSC-HVDC

Modular multilevel converter(MMC)is a relatively new and promising topology,which has gained a lot of interest in industry in the recent years due to its modularity,scalability,reliability.Its characteristic of modular design can easily adapted for applications that require different power and voltage levels.This dissertation aims to improve the reliability of this system by studying the control strategies of MMC and MMC-HVDC systems.First,the topology and the basic operating principles of Sub-Module(SM)are introduced.Then two control strategies for pre-charging of capacitors were analyzed.What's more,this dissertation compared several modulation strategies that are commonly used in system,and describe their advantages and disadvantages respectively.According to the mathematic model,this paper make clear the loss composition of MMC system,and give a formula to calculate the switching losses which take a large percentage of total losses.By optimized the traditional capacitance voltage sorting algorithm,an improved sorting algorithm is put forward in order to reduce the switching frequency and switching losses.Then a simulation model was provided to realize and configure the control strategies.Second,the mathematic model circulating current was analyzed based on the conservation of energy,and circulating current suppressing controller to eliminate the inner balancing currents was put forward.Then Combined with the characteristics of MMC,the thesis analyzed the control method in MMC based VSC-HVDC system.The control strategies were divided into system layer,converter layer and valve control layer in the HVDC system.The controllers and the system in the last two layers discussed in detail in the thesis,a simulation model was used to verify the feasibility of control strategies.Finally,a three-phase MMC experimental platform was build,the paper presents the schematic diagram of the main unit of the SM.The controller consists of DSP and FPGA,DSP is main controller,FPGA is auxiliary controller.The software and hardware design of the platform were elaborated.The control strategy discussed in this thesis was implemented on the platform.The experimental waveforms were presented and the validity of theoretical analysis were demonstrated.