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Circulating Current control of a Modular Multilevel Converter (MMC) with State Feedback Controller and Harmonic Current Suppression

This paper proposes a circulating current control method for the modular multilevel converter (MMC) in order to suppress circulating current harmonics. The proposed method employs compensation technique in the internal control loop of the MMC. Performance variations arise due to the flow of load and circulating currents through the arms of the MMC that usually result in voltage disturbance on the submodule capacitor voltages and produce harmonics in the circulating current. The implementation of typical control methods based on PI-controllers with or without plug-in controllers such as resonant controllers and repetitive controllers in the feedback loop depends on the bandwidth of the MMC current controller. The bandwidth is limited due to the limited switching frequency of the MMC. Therefore, good performance at high frequencies outside the bandwidth cannot be achieved. The proposed method improves the performance of the system by providing a current compensation which is achieved through a feedforward current outside the feedback loop of the controller in response to the voltage disturbance/capacitors voltage variation. This is achieved by supplying opposing current harmonics to the circulating current harmonics without the limitation of the bandwidth. State-feedback control is employed to implement the circulating current control and simulations are carried out to test the proposed architecture.