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Dual Switching-cycle Balancing Control for the Modular Multilevel Converters

Modular multilevel converters (MMC) with the conventional control are subject to large capacitor voltage ripples, especially at low-line-frequencies. It has been proved that by introducing multiple changes of circulating current in one switching cycle, the capacitor voltage can be balanced in a single switching period. It fundamentally alternated the MMC characteristics, shifting the MMC from a long-deemed ‘line-cycle balancing’ converter to a ‘switching-cycle balancing’ converter. Yet the control is at the expense of high complexity and has difficulty with scalability. To address the challenge, the paper proposes a novel scheme, where the capacitor voltages are balanced in two adjacent switching cycles. The control complexity therefore becomes entirely independent of MMC cell numbers. The scheme is verified under both dc-dc and dc-ac operations using computer simulations.