Circulating Current Control in MMC Under the Unbalanced Voltage

This paper proposes a control method for circulating currents in modular multilevel converters (MMCs) under unbalanced voltage conditions. Under balanced voltage conditions, only negative-sequence circulating currents exist. Consequently, the conventional method has considered only negative-sequence circulating currents in MMC. However, under unbalanced voltage conditions, there are positive-sequence circulating currents, zero-sequence circulating currents, and negative-sequence circulating currents in MMC. Thus, under unbalanced voltage conditions, a control method should consider all of these components. This study adopts a dual vector current controller as an ac-side current controller to reduce the ac-side active power ripple under unbalanced voltage. It analyzes the effect of the unbalanced voltage on circulating currents in MMC and then proposes a control method considering each component of circulating currents under the unbalanced voltage. The effectiveness of the proposed controlling method is verified through simulation results using PSCAD/EMTDC.