Small-Signal Instability of PMSG Wind Farm Caused by the PLL Under the Condition of Near Strong Open-Loop Modal Resonance

Abstract

This paper investigates the small-signal instability risk brought about by a phase locked loop (PLL) in a grid-connected PMSG wind farm. The investigation is based on a closed-loop interconnected model, wherein, the PLL and the remainder of power system (ROPS) are modeled as two separate open-loop subsystems. Analysis indicates that the closeness of two open-loop oscillation modes separately from the PLL and ROPS subsystems on the complex plane is the modal condition of near strong open-loop modal resonance (NESMOR). When the bandwidth of a dynamic component of the wind farm is close to the bandwidth of the PLL, it is possible that the NESMOR occurs between the dynamic component and the PLL. Under the condition of NESMOR, dynamic interactions introduced by the PLL degrade the small-signal stability of the grid-connected wind farm. Under the condition of weaker grid connection, the impact of NESMOR becomes stronger to lead to more degradation of system small-signal stability. Study cases are presented to demonstrate and validate the analysis and conclusions made in the paper.