The control method of modular multi-level converter based on low and high frequency circulating current

Abstract

The modular multilevel converter (MMC) is attractive for high-power applications because of the advantages of its high modularity and high power quality. This paper analyze the low and high frequency circulating current in MMC based on the phase-shifted carrier-based PWM(PSC-PWM) scheme. It can draw such conclusion that the DC component of circulating current can control the total sub-module capacitors voltage in one phase leg, while the fundamental frequency component can control the sub-module capacitors voltage difference between the upper and lower arm, and the switching frequency component can balance the sub-module capacitors voltage by reallocating the PWM pulse. The second-order frequency component can be eliminated by changing the modulation wave. A control method of MMC based on low and high frequency circulating current is proposed. This method solved the problems of voltage balance control in light load condition, simplifying the control system structure and increasing the system stability. Simulation results confirm the effectiveness of the proposed method.