A Generic Grid-forming Control Structure for Modular Multilevel Converters

The grid-forming control is recognized as an effective method to cope with the power grid stability problems with high renewable power penetration. The application of grid-forming control in modular multilevel converter-based high-voltage direct-current (MMC-HVDC) transmission systems is a promising trend. However, the MMC usually needs to switch between different control modes, such as DC voltage control, active power control, AC voltage control, etc. Conventional grid-forming control structures are hard to meet the above requirements. To solve this problem, a generic grid-forming control structure is proposed in this paper. The stored energy in the submodule capacitors of the MMC is utilized to construct the grid-synchronization mechanism. On this basis, a multi-variable control structure is applied on the DC side, making it easy to switch between different operation modes. Finally, simulation results in PSCAD/EMTDC show the effectiveness of the proposed control structure.