Seamless Transition Control for Modular Multilevel Converters When Inserting a Cold-Reserve Redundant Submodule

Redundant submodules (SMs) are always required to ensure the reliability of the modular multilevel converter (MMC). In this letter, a new control method for MMC is proposed to seamlessly ride through the period when bypassing a faulty SM and inserting a redundant SM. With the proposed method, the redundant SMs can be arranged in cold reserve mode so as to reduce the conduction losses, and when faced with SM failures, bypass of the faulty SM and insertion of the redundant SM will incur no inrush current problems. Besides, the capacitor of the newly inserted SM can get linearly charged. Validity of the proposed control method is verified experimentally based on a single-phase MMC prototype.