In recent years, the modular multi-level converter (MMC) has been widely used in high and medium voltage DC transmission systems because of its topological advantages. However, for an MMC with a two-stage model predictive control (TSMPC) method, it is difficult to precisely and reasonably design the weighting factor in the cost function. Here, an improved TSMPC method is proposed which not only can avoid choosing the weighting factor for both first and second stage control but also can raise the output voltage level (OVL) to 2N + 1 without increasing computation burden. The discrete-time mathematical model of the MMC is first derived. Two circulating current factors are introduced to calculate the optimal number of submodules (SMs) of the upper and lower arms in the first stage control. Secondly, the second stage control calculates the optimal number of SMs through the superior control and forms the optimisation array by adding or subtracting one SM. Then the objective function is developed, and the SMs with the minimum value of the objective function are selected for the final input. Finally, the algorithm of reducing switching frequency (RSF) is applied to achieve the balance of the SM capacitor voltage. The simulation and experimental results verify the effectiveness of the proposed method.
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