Research Of The Control Strategy On Modular Multilevel Converter Based STATCOM

Large-scale power grids and long-distance transmission are increasinglyapplied and a large number of high-voltage, high-power nonlinear devices widelyused nowadays. They bring some convenience for people, while bring some powerquality problems at the same time. Power system stability is directly related topower grid reactive power. STATCOM(Static Synchronous Compensator，STATCOM) is one of the main memberships of FACTS, play an important role incompensating the reactive power in the grid and improving system’s quality.Modular Multilevel Converter (MMC), with its highly modular structure, issuitable for high-voltage and high-power fields. It is conducive to reduce devicefailure loss and realize redundancy. By controlling the inverter, active and reactivepower can be exchanged with power system, so it can be applied to high powerreactive compensation apparatus. Compared with the traditional reactive powercompensation equipment, MMC based STATCOM can compensate reactive powerquickly, and operate more widely. The harmonic pollution is also smaller.The current detection method is analyzed in this paper. The operationprincipal of MMC is analyzed and calculated in this paper, as well as themathematical model in Modular Multilevel Converter Based STATCOM. Studiesshows that the circulating current is caused by the voltage difference between theupper and lower bridge arm, and unbalanced power exchanged. Research revealedthat the total energy stored in capacitances in each arm of the MMC can becontrolled by the circulating current icir. The capacitance in each sub-modular canbe balanced by the suppression of this circulating current. A voltage balancingstrategy with circulating suppression controller was put forward based on carrierphase-shifted SPWM. Furthermore, a control tactic for Modular MultilevelConverter Based STATCOM with this voltage balancing strategy was proposed inthis paper. Finally, the control strategy for MMC-STATCOM is confirmed bysimulation on Simulink platform and the control system is worked out. Thehardware control system and sub-module programming method are also been put forward. The correctness and effectiveness of the proposed strategies are verifiedby the simulation.