Critical SCR for the stability of VSC connected to weak grid

Voltage source converter (VSC) has been widely employed in power system. However, when a VSC is connected to weak grid, its stability is challenged. In this paper, a VSC-grid interaction model in sequence domain is proposed. The critical short-circuit ratio (CSCR) for the stability of grid-connected VSC is derived. Furthermore, based on CSCR, the transmission capacity limitation of grid-connected VSC is estimated and quantified. It is found that the CSCR and the maximum power transfer of grid-connected VSC are determined as a function of phase-locked loop (PLL) bandwidth. The theoretical findings are verified by the Generalized Nyquist Stability Criterion (GNC) and time-domain simulation.