Control of Modular Multilevel Converters as Virtual Synchronous Machines

Modular multilevel converters (MMC) have been a research focus in high-voltage power conversion, covering modulation techniques, capacitor voltage balancing and circulating current control. In this paper, a control method is proposed to control MMCs to behave like virtual synchronous machines (VSM). It can be operated in the set mode and the droop mode. The dynamics, implementation and operation of the VSM-like MMC arc introduced and analyzed. In particular, how the VSM control algorithm works with the circulating current control in MMC is proposed. In this paper, three proportional-resonant (PR) controllers are adopted to inject second-order harmonics to the MMC in order to suppress second-order circulating current. The simulation results are provided to verify the frequency and voltage droop mechanisms, the circulating current suppression of the VSM-like MMC.