Analysis and Damping Control of Small-Signal Oscillations for VSC Connected to Weak AC Grid During LVRT

The instability issues of grid-connected voltage source converters (VSC) may easily occur during low voltage ride-through (LVRT), especially when connected to a weak ac grid. In this study, a small-signal model of the grid-connected VSC system was developed to deal with the stability problems during deep voltage sags. Based on the model, the interaction between the phase-locked loop (PLL) and current control has been illustrated. In addition, the eigenvalue and modal analysis method was employed to investigate the influencing factors of the VSC system stability, which include the bandwidths of the PLL and current control loop, grid strength, and voltage sags. Furthermore, on the basis of the interaction between PLL and current control loop, a novel additional damping controller that is placed in the active current control loop was proposed and designed. Finally, experiments were conducted to verify the theoretical analysis and proposed control strategy for enhancing VSC system stability during LVRT with a high impedance grid connection.