Fractional-Order Composite Control for Fractional-Order Modular Multilevel Converters

The existing basic control system of fractional-order modular multilevel converters (FO-MMCs) is mainly designed for ideal operating conditions and exhibits limited adaptability under grid unbalanced conditions. To address this issue, this paper proposes a fractional-order composite control system for a FO-MMC. The proposed system consists of fractional-order positive-sequence and negative-sequence control loops. The positive-sequence control loop includes a power outer-loop fractional-order proportional-integral (FOPI) controller and a fractional-order decoupled FOPI positive-sequence current inner-loop controller. The negative-sequence control loop adopts a fractional-order decoupled FOPI negative-sequence current inner-loop controller. Combined with three sequence current reference calculation strategies, the proposed system achieves three control objectives: active power oscillation elimination (APOE), reactive power oscillation elimination (RPOE), and balanced positive-sequence current output (BPSC). A FO-MMC simulation model was established based on the MATLAB/Simulink platform, and the proposed control system was verified. The results demonstrate that the proposed control system achieves accurate regulation of positive- and negative-sequence currents. Under different control objectives, the system exhibits excellent dynamic response and steady-state performance, providing a theoretical basis for stable FO-MMC operation under unbalanced grid conditions.