A Study on Small-Signal Stability Improvement of a VSG-Based GFM Inverter under Strong-Grid Conditions Using Virtual Impedance

Grid-forming inverters (GFMs) can contribute to enhancing the voltage and frequency stability of power systems. However, small-signal instability accompanied by low-frequency oscillations may occur depending on grid strength and control parameters. This paper develops a state-space model and a dq-axis admittance model for a GFM inverter employing virtual synchronous generator (VSG) control. Based on these models, the small-signal stability of the multi-input multi-output (MIMO) system is analyzed using the generalized Nyquist criterion. The accuracy of the developed model is validated by comparing the admittance derived from the state-space model with that extracted from PSCAD simulations. It is also confirmed that the oscillation frequency of the dominant mode calculated from the state-space model agrees with that observed in the PSCAD active power response. In addition, eigenvalue trajectories associated with variations in the short-circuit ratio (SCR) and damping coefficient are analyzed, demonstrating that the stability margin tends to decrease as grid strength increases. To mitigate the deterioration of stability under strong-grid conditions, virtual impedance is applied, and the admittance characteristics, generalized Nyquist plots, and active power responses before and after its application are compared. The results demonstrate that virtual impedance improves the oscillation damping performance and small-signal stability of the GFM-VSG system under strong-grid conditions.