Influence of PLL on Stability of Interconnected Grid-Forming and Grid-Following Converters

This letter analyzes the impact of a phase-locked loop (PLL) on small-signal stability in a system of interconnected grid-forming and grid-following (GFL) converters. It identifies an undiscovered instability phenomenon, i.e., the system is prone to unstable when the bandwidth of PLL used with GFL converter is low. Experiments validate the theoretical analysis.