Analysis and Control of Modular Multilevel Converters Under Unbalanced Conditions

This paper investigates the contents of submodule voltage ripples, circulating currents, and internal converter voltage in modular multilevel converters (MMCs) theoretically. The operation of MMCs is studied under asymmetry of the upper and lower arm, as well as the unbalanced ac system. In three-phase MMCs, the analysis shows that the second harmonic circulating currents will be asymmetric under an unbalanced ac system and can be decomposed into positive-, negative- and zero-sequence parts. The positive- and negative-sequence components affect neither the ac side nor the dc side of MMCs while the zero-sequence components will flow into the dc side, aggravating the power fluctuations of the dc side. In order to solve this problem, a new controller based on the instantaneous power theory and proportional-resonant scheme is designed. Simulations with a detailed switching model on the PSCAD/EMTDC platform verify the theoretical analysis, and demonstrate that the proposed controller eliminates the active power fluctuations and suppresses the harmonic circulating currents as well.