Impact of PLL control on small-signal stability of wind DFIGs connected to weak grids

Research into doubly-fed induction generator (DFIG) technology has yet to solve the instability that arises when DFIGs are connected to weak grids. To investigate this problem, this work used the modal analysis technique to study the impact of the phase-locked loop (PLL) control on the small-signal stability of wind-driven DFIGs and examine the sensitivity to the grid-impedance. Using a highly-detailed model, this study explains how stability can be guaranteed in terms of the ratio between the bandwidth of generator current controllers and the bandwidth of the phase-locked loop controller. Finally, the study provides practical recommendations for PLL design, based on the research findings.