High-Symmetry Operation Control Algorithm for Receiving-End MMC Arm Current Oriented to Unidirectional Power Transmission

To address the asymmetric arm-current loading in the receiving-end modular multilevel converter (MMC) under unidirectional power transmission, this paper proposes a high-symmetry operation control algorithm for bridge-arm-current redistribution and circulating-current coordination. In conventional receiving-end MMC stations, the active power direction remains almost unchanged for a long period, while the AC-side reactive support and DC-side voltage regulation are simultaneously required. This operating pattern can produce unequal current distribution between the upper and lower arms, an enlarged second-harmonic circulating current, uneven submodule capacitor energy, and increased thermal stress of power semiconductor devices. To address these problems, the arm-current components are decomposed from the perspectives of DC current, AC phase current, and internal circulating current. Using that decomposition, a symmetry-oriented current reference generation method is established, and a coordinated controller combining arm-current balancing, capacitor-voltage energy equalization, and circulating-current suppression is designed. The proposed method does not change the external active-power transmission target but adjusts the internal current path of the receiving-end MMC to improve the symmetry of arm currents and reduce device stress. Simulation and HIL tests directly evaluate the controller under rated power, power step, reactive power support, DC voltage fluctuation, and unbalanced grid conditions. The resulting data indicate that the proposed algorithm can effectively reduce the arm-current asymmetry index, suppress low-frequency current fluctuation, and improve the steady-state and dynamic operation reliability of the receiving-end MMC.