Control Strategies And Fault Isolation Methods Of MMC-HVDC Transmission System

Modular multilevel converter has many advantages such as modular design, good harmonic performance, and so on. It has become the main technological scheme of the voltage source converter topology. The control strategy and the fault isolation method of MMC are two most critical techniques of MMC-HVDC transmission, which ensure the safe and stable operation of the whole transmission system. This paper analyzes the control strategies of the MMC-HVDC system and the DC fault isolation method theoretically and both methods are tested in simulation environment.This paper first introduces the development of VSC-HVDC transmission technology, summarizes the existing research of MMC-HVDC, and expounds the significance of this study.Secondly, the topology, working principle and operation characteristics of FBMMC are analyzed. Its mathematical model is established, the modulation strategy is determined, and the basic parameters of MMC are analyzed and designed.Then the control strategy of MMC-HVDC system have been studied, and the inner and outer loop controller is established. To avoid the fluctuations of sub module capacitor voltage, capacitor voltage balance control strategy is determined. According to the inhibition mechanism of circulation, the corresponding circulation control strategy is proposed. Combined with the recent level of approximation modulation strategy, the FBMMC trigger control system which includes modulation module, capacitance voltage modules and the rotation of IGBT turn-on module is given. The excited method and self-excited method of MMC are studied separately. A two terminal 21 levels MMC-HVDC system is modeled in PSCAD/EMTDC to simulate the system startup, normal operation and control strategies.Finally, the failure mechanism of HBMMC is analyzed. DC fault isolation method is studied, which mainly divided into DC circuit breaker isolation method, AC circuit breaker isolation method and the isolation method based on converter itself. Using FBSM as the basic topology, the mechanism of isolating DC fault current and control strategy of FBMMC-HVDC system are studied. The capability of DC fault isolation is verified in PSCAD/EMTDC.