Unbalanced PCC voltage regulation with positive- and negative-sequence compensation tactics for MMC-DSTATCOM

Unbalanced voltage fluctuation at the point of common coupling (PCC) is a challenging power quality problem in power distribution system. This study presents an unbalanced PCC voltage regulation scheme with positive- and negative-sequence control for modular multilevel converter (MMC) in medium-voltage distribution static synchronous compensator (DSTATCOM) application. The proposed positive-sequence control is utilised to regulate the positive-sequence component of PCC voltage to rated value, while the negative-sequence control aims to cancel out the negative-sequence component of PCC voltage completely. Compensation commands whether in positive- or negative-sequence control are generated from unbalanced PCC voltage without utilisation of unbalanced load currents. DC circulating currents transferring active power among three phases of MMC are also analysed to illustrate power balance issue resulting from negative-sequence control. Then comparison of negative-sequence compensation capacity between MMC and full-bridge cascaded converter with delta-connection is illustrated. Experiment results acquired from a downscaled 85 V/1.5 kVA apparatus validate the effectiveness of the proposed control scheme.