Broadband Circulation Suppression of Modular Multilevel Converter under Voltage Fluctuation

Along with the rapid development of power industry, modular multilevel converter (abbr. MMC) has been increasingly used in DC transmission, distribution, grid-connection of new energy, and so on. However, when voltage fluctuation occurs at the DC side or AC side of MMC, circulating current will be generated on the bridge arm. To reduce the influence of circulation on MMC, firstly, the capacitance voltage fluctuation of the sub-module was derived by coupling the upper and lower bridge arm currents with the switching function, and the transfer law of the circulating current in phase sequence and frequency was obtained. Secondly, based on quasi fast repetitive control (abbr. Q-FRC) and combining with quasi proportional resonance (abbr. QPR) controller, a circulating current suppression controller was designed to achieve the effects of wide-band suppression of circulating current and key suppression of specific frequency. Finally, the analysis on the PSCAD simulation platform for three abnormal conditions, i.e. DC side bus voltage fluctuation and AC side voltage fluctuation were simulated by the PSCAD simulation platform, thus, the correctness of the theoretical derivation and the effectiveness of the proposed suppression measure are verified.