A novel circulating current controller for modular multilevel converter

The system losses and working performance of the modular multilevel converter (MMC) are directly affected by the harmonics in the circulating current. The inherent relation between the AC-side current, the leg current and the circulating current is first directly derived, which reveals that there always exists double-frequency harmonics whether under balanced or unbalanced grid conditions. Then, a novel circulating controller based on proportional-integral-resonant (PIR) control is proposed for effectively suppressing the harmonics in the circulating current, without the double-frequency negative-sequence transformation and the phase decoupling control. A MMC-HVDC simulation of a two-station model containing the PIR is created based on PSCAD/EMTDC. Simulation results have verified the availability of the control strategy when compared to the results from the circulating controller under double fundamental-line frequency negative sequence rotational frame.