Improving the operation of the modular multilevel converters with model predictive control

This paper proposes a simple variable nearest level control-model predictive control (VNLC-MPC) strategy which effectively enhances the modular multilevel converter (MMC) operation. Different from the conventional nearest level control (NLC), the proposed technique selects the rounding method for each arm according to the cost function value which corresponds to the weighted sum of the output current tracking error and the AC circulating current. In this way, the NLC is efficiently combined within the frame of MPC. To illustrate the effectiveness of the VNLC-MPC strategy, simulations are conducted on a three-phase MMC power system. Results have verified the capability of the VNLC-MPC in tracking the output current reference with low THD and reducing the circulating current, as well as showing high dynamic during the transient. Further tests show that by introducing one constraint to the capacitor voltage balancing scheme, the switching frequency of the proposed method can be greatly reduced without influencing the overall performance.