Controllers for eliminating the ac components in the circulating current of modular multilevel converters

Control of the circulating currents in a modular multilevel converter (MMC) is an integral part of its operation as the circulating currents have a significant impact on the rating of the power devices, losses and also capacitor voltage ripples. This study proposes two methods to control the circulating currents in the phase-legs of the MMC to their minimum dc value through elimination of the low-order harmonic components in the circulating current. The first method acts directly on the ac harmonics of the circulating current allowing the dc component to establish naturally, whereas the second one directlym controls the current to its dc reference. Simulations and experimental results from a laboratory prototype demonstrate the operating principles of the two methods and validate their performance.