Novel Virtual Synchronous Control of VSC-based HVDC Transmission for Interarea Connection

PLL-based grid-following control might induce negative damping and result in the instability of an overall meshed AC/DC system in the case of a low grid short circuit ratio or the high bandwidth control of PLL. To overcome the drawbacks of conventional PLL-based control, this paper proposes novel virtual synchronous control for VSC-based HVDC systems without the requirement of PLL for the interarea connection of power grids. In the proposed control method, the droop property between DC-link voltage and the frequency of a receiving-end system is artificially constructed. As a result, emulated inertia support can be provided from the DC-link capacitor for the system under severe system frequency deviations. Moreover, a sending-end AC system can sense the frequency disturbance in the receiving-end system without remote communication by utilizing the local DC-link voltage and current variations in the VSC-based rectifier. In addition, the sending-end system can provide the primary frequency control for the disturbed system on the other side by fully emulating the primary frequency regulation of a conventional synchronous generator. Compared with the conventional PLL-based control of VSC, the proposed control strategy is different in that it fully utilizes the grid-forming and grid-support potential of VSC for the disturbed AC system without relying on PLL and remote communication. Furthermore, the stability of the receiving-end system is considerably improved by the proposed scheme. The effectiveness of the proposed control method is validated through the simulation of abrupt load variations using PSCAD/EMTDC.