Unified Grid-Forming Control for MMC-MTDC Systems with Renewable Power Integration

Using modular multilevel converter-based multi-terminal high voltage direct current (MMC-MTDC) transmission to integrate large-scale renewable power into the main power grid is an important development trend in China. To cope with the stability issues under weak grid conditions, different grid-forming control strategies have been proposed for the MMC in MTDC transmission systems. These control strategies have different structures due to different control targets and are difficult to unify. To solve this problem, a novel grid-forming control structure is proposed in this paper. The MMC emulates the synchronous generator (SG), and the submodule capacitor voltage emulates the SG rotor speed. On this basis, by utilizing the modulation control degree freedom, a unified DC-side controller is proposed to realize different control targets. Finally, the simulation model of a 4-terminal MMC-MTDC transmission system with renewable power integration is established in the PSCAD/EMTDC. The effectiveness of the proposed strategy is verified by the simulation results.