Low-frequency suppression strategy based on MPC without common-mode voltage increases

Modular multilevel converters (MMCs) have attracted a great deal of attention in medium-voltage motor drive applications. However, the excessive fundamental frequency submodule (SM) capacitor voltage ripple at low frequencies is a major technological challenge. In this paper, an SM capacitor voltage fluctuation suppression strategy based on model predictive control (MPC) is proposed. A cost function is employed to replace multiple PI controllers, which makes the control strategy simpler, and avoids the process of the high-frequency injection. To avoid the common-mode voltage (CMV) generated by the proposed suppression strategy, a flying capacitor is added between the upper and lower arms of a traditional MMC. The MPC strategy is divided into two stages. In the first stage, the SM input number of each arm is obtained through an output current prediction model. In the second stage, the optimal switching states are output through a model for SM capacitor voltage fluctuation suppression. When compared with traditional MPC, the design of the stepwise MPC strategy and finite control set omits the design of partial weight factors, which reduces the number of calculations of the SM capacitor voltage fluctuation suppression strategy. Simulation and experiments are carried out at 5 Hz to verify the effectiveness of the proposed strategy. The obtained results show that the proposed strategy can suppress low-frequency SM capacitor voltage fluctuation without increasing the CMV.