Zero-Cost Closed-Loop Discharging Method for Modular Multilevel Converter Submodules Without External Circuits

A novel submodule capacitor discharging control strategy is proposed and implemented for modular multilevel converters (MMCs). By sequentially controlling the arm currents of an MMC, all submodule capacitors are discharged rapidly through internal arm resistors without additional devices and circuits, resulting in improved performance and reduced cost. A closed-loop control method is investigated for the effectiveness of the new control strategy. A comparison study is evaluated between the proposed method and the traditional methods using bleeding resistors, auxiliary resistors, and load resistors. Validated by the simulation and experiment, the proposed method has the maximum benefits including zero hardware cost, fast discharging time, and negligible load current impact.