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In this paper, an improved modular multilevel converter (MMC) control strategy based on one-cycle control (OCC) is proposed. With the introduction of OCC, the stability, rapidity and disturbance rejection of MMC is greatly improved. Based on the volt-second equilibrium, the control is distributed to every single submodule (SM). For further stability, an unbalanced OCC strategy is designed based on feed-forward control for negative sequence voltage with no need for measuring the positive and negative current sequence, considering single phase to ground fault and the second harmonic ripples of the DC voltage. As the switch frequency of every SM is different, the virtual loop mapping is used in this paper to balance the capacitor voltage with less hardware dependence. And a new simplified structure is also proposed with RS flip-flop abandoned. At last, simulation and experiment results prove the validity of the control theory.

An Improved MMC Control Strategy with Single-Phase to Ground Fault-Tolerance Capability