A virtual circuit based circulating current suppressing strategy to enhance the stability of single-phase MMC grid-connected system

In order to enhance the stability of grid-connected modular multilevel converters (MMC) system, which has become a prominent topology as an alternative to conventional voltage source converter in applications for high voltage, a novel singlephase circulating current suppressing strategy is proposed. This paper studies the mechanism and model of circulating currents of a single-phase MMC system at first, then presents a circulating current eliminating method using virtual circuit and PARK coordinate transformation with simple proportional-integral controllers, avoiding sophisticated mathematical calculation and control scheme. Simulation results demonstrate the correctness of the presented method, which can not only minimizes the circulating currents, but also stabilizes the active power and improves the quality of output voltage. Consequently the stability and reliability of grid-connected system are intensified, which is of significance to the construction of medium and high voltage grid.