A New Control Strategy for Modular Multilevel Converter Operating in Quasi Two-Level PWM Mode

Modular multilevel converter (MMC) is difficult to operate when driving a motor with large constant load torque at low speed, due to the large voltage fluctuation of submodule (SM) capacitors on this condition. Quasi two-level PWM is an alternative control method to solve this problem. This paper proposes a new control strategy to implement quasi two-level PWM for a three-phase MMC inverter. Arm current commutation is realized by control delay time of each PWM signal, and voltage of SM capacitors are kept balanced by change the connecting relationship between PWM signals and SMs, without requirements of a high-frequency PWM control. The control system is verified by simulation results and compared with high frequency circulating current injection method under same load condition. Although the total harmonic distortion (THD) is relatively large due to the sacrifice of multilevel operation, the voltage fluctuation of proposed method is only 28.5% of conventional circulating current injection method, and what is more, the arm current stress is also reduced by 24%.