Analysis and Mitigation of Power-Transfer-Dependent Small-Signal Instabilities in Interconnected GFM-GFL Systems

In interconnected systems comprising grid-forming (GFM) and grid-following (GFL) converters, different types of small-signal instabilities may occur. However, an analytical interpretation of their underlying mechanisms and dependence on active power transfer remains lacking, hindering the understanding of instability phenomena and the determination of stable operating ranges. To address this gap, this article proposes a single-input single-output model for interconnected GFM-GFL systems. Based on this model, the mechanisms and power-transfer dependence of two distinct types of instabilities are analytically interpreted: one arises from the interaction between the GFM converter's inner voltage control and the GFL converter's phase-locked loop, with instability risk intensified when the GFL converter transfers higher active power to the GFM converter; the other arises from the interaction between the GFM converter's outer active power control and the GFL converter's phase-locked loop, with instability risk intensified under higher active power transfer in either direction. Furthermore, to ensure effective mitigation of these instabilities across the full range of power transfer, analytical boundaries for critical converter control parameters are derived. Finally, the theoretical analyses and the derived parameter boundaries are validated through real-time simulations and experiments.