Research on the Grid-Forming Control Strategy of the Receiving-End MMC and the Autonomous Inertia Response Method of the MMC-HVDC System

Aiming at the issues of inertia deficiency and weak grid stability when the high-voltage direct current (HVDC) transmission system based on modular multilevel converter (MMC) is connected to the grid using a phase-locked loop (PLL), a grid-forming control strategy suitable for the receiving-end MMC is proposed. This strategy includes a PLL-free gridconnection control strategy based on the dynamic selfsynchronization of sub-module capacitor energy and a damping enhancement strategy based on the active power difference between the AC and DC sides, endowing the receiving-end MMC with the characteristics of a voltage source grid connection. A typical point-to-point MMC-HVDC system is constructed using the PSCAD/EMTDC simulation software to verify the effectiveness of the proposed control method under the condition of sudden power changes in the wind farm.