Analysis and Injection Control of Zero-sequence Voltage and Circulating Current for MMC with Active Power Filter

Modular multilevel converter (MMC) is considered as the next generation topology for medium and high voltage applications. Fluctuation-suppression of sub-module (SM) capacitor voltages is significant for MMCs. This paper proposes an improved 3rd-order frequency zero-sequence voltage and circulating current injection control method to suppress the capacitor voltage ripples. In this method, the amplitude of injected voltage is 1/6 times that of the output voltage and the amplitude of injected circulating current is varying according to the instantaneous value of the output voltage and current, which will improve dc voltage utilization and reduce capacitor voltage ripples. To further reduce the capacitor voltage ripples, circulating currents and power losses, an additional active power filter (APF) that diverts the phase pulsating powers is paralleled between the NTh SM of the upper arm and the 1sT SM of the lower arm in each phase of traditional MMC. A down-scaled MMC prototype was built, and the experimental results verify the effectiveness.