Hybrid synchronization based frequency support control for renewable energy sources integrated MMC-HVDC with capacitor voltage control

Modular multilevel converters based high-voltage direct current (MMC-HVDC) transmission is widely used for renewable energy sources (RESs) integration. A novel hybrid synchronization based frequency support control is proposed for RES integrated MMC-HVDC to overcome instability risk of phase-locked loop (PLL) based grid-following (GFL) control. Three contributions are made as follows. (1) By adopting the revised DC voltage regulation loop and hybrid synchronization with AC and DC voltage for synchronization, the proposed control realizes simultaneous grid synchronization and DC voltage adjustment according to the synchronized grid frequency for the receiving end MMC. (2) The sending end frequency is adjusted by lead-lag block seriated hybrid synchronization loop with DC voltage drop compensation. Thus, RESs can provide frequency support for the receiving end with extra control freedom. (3) The capacitor voltage can be adjusted by modifying the inserted submodule number to provide frequency support. Meanwhile, system modelling is presented for the MMC-HVDC system with RESs and synchronous generator at the sending and receiving end for the parameter design. Moreover, the coupling between AC and DC side for the proposed control is analysed. Finally, simulations verify the fast and stable frequency support over the conventional PLL based GFL control.