Research On Control Scheme For Low Frequency Operation Of The Modular Multilevel Converter

Multi-level converters have been widely used in high power motor driveapplications with the purpose of improving motor operation efficiency. Recently,Modular Multi-level Converters(MMC) is getting more and more attention for its highmodular integration and easy expansion of voltage class, but there is still a lot of workto do on MMC. This thesis mainly studies carrier phase-shifted modulation, controlstrategy of sub-modular capacitor voltage, control method for low frequencyoperation and experimental prototype.Firstly, topology of MMC and operation theory are presented in detail in thethesis, and makes a comparison between5-level carrier phase-shifted modulation and9-level carrier phase-shifted modulation which can output more levels and get lowerTHD; then the realization of sub-modular capacitor pre-charging and control strategyof voltage balance are analyzed, and the feasibility is simulated in the platform ofMatlab/Simulink.Then, this thesis establishes the equivalent mathematical model of MMC. Lowfrequency operation characteristic is analyzed based on mathematical model and thevoltage fluctuation of sub-modular is relevant to output current and frequency. Inorder to suppress the voltage fluctuation, a control method is proposed by injectinghigh frequency zero-sequence voltage and phase-to-phase circulating currents. Thefeasibility and validity of the low frequency operation controlling strategy is verifiedaccording to the simulation results with RL and motor load.Finally, experimental prototype is built which contains the main circuit andcontrol system design. The parameters of main circuit is calculated and designed aswell as the software of control system. The carrier phase-shifted modulation, controlstrategy of sub-modular capacitor voltage and control method for low frequencyoperation are verified and the experimental results also indicate the validity of themethod used in this thesis.