Research On Model Predictive Control Stratrgy Of Modular Multilevel Converter

Multilevel converters have been widely applied in high-voltage and high-power electrical engineering field. Modular multilevel converter(MMC) is a novel multilevel converter topology proposed recently, and MMC is a research hotspot for its advantages in high voltage direct current(HVDC) area. Two problems should be solved for MMC, which are the restrain of circular current and the balance of capacitor voltages of submodule. This thesis mainly studies on modulation strategies and control strategies for MMC, aimed at those two problems, mainly focused on model predictive control(MPC) strategy for MMC.The operation theory of MMC was analyzed, then some research were conducted in view of theory and simulation as to carrier phase shifted sinusoidal pulse width modulation(CPS-SPWM) strategy and nearest level modulation(NLM) strategy employed by MMC. Based on the mathematical model of MMC under two-phase synchronous rotary coordinates and the model of double-frequency circular current, the classical current control strategy was studied which can inhibit the double-frequency circular current.Based on the MPC strategy used for two-level converters, Finite Control Set MPC(FCS-MPC) strategies used by MMC was further studied. To solve the problem of excessive computation for FCS-MPC when the bridge arm of MMC has more submodule, Optical MPC(OMPC) strategy based on the theory of sorting of capacitor voltages was proposed in this thesis, which can reduced the number of switching states for n-level MMC to n+1. The disadvantage of OMPC strategy is that sequencing of capacitor voltages of submodule should be considered during the execution process of the strategy.As to MMC system which has larger number of submodule, a strategy called Reduced Finite Set-fast MPC(RFS-FMPC) was proposed in this thesis. As to FCS-MPC strategy, the output voltage levels are set as controllable variables and applied to create a collection, then the control structure of MMC is simplified. Not only can this strategy satisfied the three control goals that output current trace, inhibition of circular current and the balance of capacitor voltages of submodule, but also improve dynamic performance of MMC system. By simulation comparing RFS-FMPC, OMPC, FCS-MPC and classic current control strategy,obtain their advantages and disadvantages.Parts selection and calculation of the primary loop and design of the structure of MMC experimental system were conducted. In the experimental system, the control strategy proposed in this thesis is verified by experiments, and experimental results verified the validity of those strategies.