Small-signal Synchronous Stability Mechanism of Renewable Energy Transmission System Through VSC-HVDC Working as Island Mode.

In response to the demand for green and low-carbon development, China is vigorously promoting the construction of large-scale renewable energy bases. The system of transmitting renewable energy through VSC-HVDC is receiving more and more attention. In such a system, renewable energy devices usually adopt grid-following control, while VSC-HVDC adopts grid-forming control. However, the synchronous stability mechanism of this high-proportion or even 100% power electronic system is still unclear. This paper establishes a synchronization model for grid-following and grid-forming devices, focusing on the role of phase-locked loops and virtual synchronous generator loops in synchronization. Then, the small-signal stability of the system is analyzed, and the operating area that ensures stable operation is derived. The operating states with instability risks are proposed for both renewable energy devices and VSC-HVDC. Finally, the conclusions are verified by PSCAD simulation.