Precise Control Law of MMC and Its Application in Reducing Capacitor Voltage Ripple by Injecting Circulating Current

Modular multilevel converter (MMC) has been a research hotspot in the past decade in the high-voltage high-power applications. The basic working principle is analyzed in depth in this paper firstly. A precise control law is deduced to control the output voltage and the circulating current directly and precisely. Different from the conventional control law, arm inductor voltages are considered in the proposed method. It is proved that in terms of eliminating the circulating current, the effect of this control law is approximately equivalent to the conventional closed loop control with circulating current suppressing controller (CCSC). Based on this control law, the operation of MMC is further analyzed and the modulation index limitation is offered when the circulating current is eliminated. Furthermore, this paper deals with reducing the capacitor voltage ripple. After comparison, the paper gives an optimized form for single-phase MMC when only second-order circulating current is injected. The analysis can be extended to three-phase MMC. Theoretical analysis and simulations verify all the conclusions.