Impact of Unbalanced Grid Voltages on Capacitor Voltage Ripple and Capacitance Selection in AC/AC MMCs

This paper evaluates the submodule capacitor current and voltage ripple of a direct three-phase to single-phase ac/ac MMC under unbalanced grid conditions. The submodule capacitor RMS current and the necessary capacitance to maintain a desired voltage ripple are determined, which can be used to dimension the ac/ac MMC's submodule capacitors. The derived analytical expressions are evaluated under unbalanced grid conditions using three well-established current control techniques. The analysis is validated through simulations and measurements for a phase-to-ground fault.