Control Strategy of Grid Forming MMC system for Grid Voltage Sag Ride Through

Modular Multilevel Converter (MMC) based flexible DC transmission system is widely used for its advantages of flexible power control and low harmonic level. The traditional MMC adopts the grid-following control, which cannot play the role of active support during grid faults. In this paper, the virtual synchronous generator (VSG) grid-forming control can realize active support to the grid, but it is easy to overcurrent when the grid voltage sags. Aiming at the effective response to grid voltage sags, this paper analyzes the parameters affecting the fault current through expressions, and proposes a strategy to suppress the transient state overcurrent and steady state overcurrent. At the same time cooperate with the virtual impedance to realize the rapid support of the grid voltage without damaging the power electronic devices to realize the grid-forming MMC low voltage ride through.