Low-Frequency Oscillation Analysis and Auxiliary Damping Control of Virtual Synchronous Generator Systems

Abstract

Grid-forming virtual synchronous generators may experience low-frequency instability when controller settings change or network strength change. To examine this issue, a small-signal representation of a single grid-connected VSG is developed. The model incorporates power regulation dynamics, impedance emulation, cascaded voltage-current control, and transmission-line behavior. Modal characteristics are evaluated using system eigenvalues and state participation factors, with emphasis on how inertia emulation, damping coefficients, and grid short-circuit ratio influence the dominant oscillatory mode. An auxiliary controller inspired by the power system stabilizer is then designed to provide additional damping, and its output is added to the reference value of the voltage magnitude. Simulation results demonstrate that this method shifts the low-frequency mode toward a more strongly damped region, suppresses oscillations in both active and reactive power, and consequently enhances the dynamic stability of the grid-connected system.