This chapter explores the influence of power grid strength and phase-locked loop (PLL) parameters on the small signal stability of grid-connected doubly-fed induction generator (DFIG)-based wind farms. Initially, it delves into the modal analysis of grid-connected DFIG wind turbines under various operating conditions and different power grid strengths. The findings from this analysis indicate that DFIGs connected to weak grids are prone to stability issues under heavy-duty operating conditions due to PLL oscillations. The primary aim of this chapter is to elucidate the mechanisms behind PLL oscillations, identify the factors that influence these oscillations, and propose a damping solution to mitigate this instability. To achieve this, a simplified linear system model of the grid-connected DFIG wind turbine is introduced, facilitating the analysis of PLL oscillations. By employing the complex torque coefficients method and utilizing this model, the book identifies the oscillation mechanisms and the influence factors, including power grid strength and PLL parameters. To address the PLL oscillation issue, the book proposes and designs a mixed \(H_2{/}H_{\infty }\) robust damping controller for the DFIG. The effectiveness of the proposed damping controller and the accuracy of the analysis results are validated through electromagnetic transient simulations conducted on both single-DFIG systems and multiple-DFIG systems. This comprehensive investigation not only enhances the understanding of the interaction between DFIG-based wind farms and power grids but also provides practical solutions for improving stability in weak grid conditions.