Analysis of the Enhancement Effect of a Virtual Synchronous Generator on the Small Disturbance Synchronization Stability of a Grid-Following Renewable Energy Station

Grid-following (GFL) renewable energy stations may experience phase-locked loop (PLL)-dominated small-disturbance synchronization instability in weak grids, while grid-forming (GFM) devices can improve the damping of this mode. However, most current studies directly approximate GFM as an ideal voltage source without providing rigorous proof. There is also a lack of in-depth analysis on how the parameters of GFM affect its enhancement effect on the small disturbance synchronization stability of GFL. To address this gap, this paper establishes an ordinary differential equation model of the GFL/GFM hybrid system and proposes a fourth-order polynomial approximate root method suitable for polynomials with two pairs of weakly damped conjugate complex roots. Based on this method, the approximated form of the PLL damping is given, and the effect of the inertia time constant, damping coefficient, and capacity of a virtual synchronous generator (VSG) on the PLL mode damping was analyzed analytically. Moreover, it was proven that VSG with a certain level of inertia can play a role close to the ideal voltage source in improving the small disturbance synchronization stability of GFL. Finally, an electromagnetic transient simulation model was established on MATLAB/Simulink, and these conclusions were verified.