Asymmetric Mode Control of MMC to Suppress Capacitor Voltage Ripples in Low-Frequency, Low-Voltage Conditions

This paper proposes an asymmetric mode control for a modular multilevel converter (MMC) operating at low-frequency and low-output-voltage conditions. Differently from conventional symmetric operations, affected by large capacitor voltage ripples caused by the large dc-link voltage, with the proposed asymmetric mode MMC operations, one arm does not produce output current and applies the major dc-link voltage, whereas the other arm sustains almost all output current with low dc-link voltage. Variations of the power level in arms remain bounded and the MMC does not need any other power exchange technique such as high-frequency or harmonic current injection in a low-frequency/low-voltage condition to suppress the capacitor voltage ripple. The asymmetric control realization, and arm energy regulation and its optimization are presented in detail. Experimental and simulation results show that the proposed method is suitable for motor drives operating under 20 Hz.