Research And Application Of Modular Multi-Level Converter(MMC)

Modular multi-level converter(MMC)is a promising converter topology,which is suitable for high voltage and power applications,including the high-voltage DC transmission(HVDC).This paper focuses on basic principle,mathematical mode,control strategy and application of MMC.The main contents are as follows:(1)From the basic structure of MMC with half-bridge submodule topology,the basic theory of MMC is analyzed.The operating principle and mathematical model of the double-star topology MMC based on half-bridge sub-module are introduced.According to the functions of sub-module capacitor and the bridge arm inductance,the parameter design method is discussed and given.(2)Different modulation principle and characteristics are concluded.The sub-module capacitor voltage control based on nearest level modulation(NLM)and the hierarchical control for voltage balance based on independent carrier phase-shift(CPS)PWM are discussed.And each part of the hierarchical control is given,furthermore the controller is verified by the simulation results.(3)Bridge arm circulating current and its suppression method are discussed and analyzed.In practical applications,there exists unavoidable parameter unmatch between the upper and lower bridge arms,making the components of circulating current more complex and the first-order harmonic component appears.In order to suppress the circulating current,the suppression methods are applied for the MMC systems both with asymmetric or symmetric bridge arms parameters.The simulation results show that the circulating current is suppressed effectively.A MMC experimental platform is built to implement the MMC inversion and rectification experiments.The suppression method for MMC with symmetric bridge arms parameters is tested in the inversion experiment and the suppression method for MMC with asymmetric bridge arms parameters is proved in the rectification experiment.(4)The application of MMC in HVDC is discussed.This thesis has introduced the principles of control strategies of MMC-HVDC connected with wind farm and its simplified equivalent structure.The rectifier-side MMC station applies constant AC voltage control and inverter-side MMC station applies constant DC voltage control.The control strategies are proved through the simulation results.