With the increasing integration of wind turbine connection into power grids, the inductive impedance of the grid has correspondingly increased, which reduces the power transmission capacity of transmission lines. Series compensation using capacitors has been widely adopted as a more economical means. However, this practice introduces a heightened risk of sub-synchronous oscillations (SSO), particularly in systems employing doubly-fed induction generators (DFIGs). In such systems, the rotor-side resistance is influenced by slip, potentially resulting in a negative resistance effect on the stator side, which can lead to system divergence.

This paper provides an in-depth investigation into the mechanisms underlying system oscillations in DFIG-based systems under sub-synchronous resonance (SSR) conditions. Mathematical models are developed to analyze the impact of DFIG dynamics and rotor current control on SSR damping based on impedance analysis. To address the issue, a novel method—sub-synchronous stator flux injection—is proposed and demonstrated to effectively enhance SSR damping.

Theoretical analyses are validated through impedance scaning and simulation-based tests involving series capacitor compensation. The results indicate that the proportional gain kp in the current controller is functionally equivalent to rotor resistance, and an increase in kp​ adversely affects SSR damping. In contrast, the integral gain ki​ exhibits negligible influence on SSR damping, a conclusion corroborated by the impedance scan outcomes.

Furthermore, the study highlights the critical role of mutual inductance in determining SSR damping characteristics. The proposed sub-synchronous stator flux injection method effectively reduces the equivalent mutual inductance, which reduecs the effect on the stator side due to negative resistance on the rotor side, thereby substantially enhancing system damping on the grid side. The simulation results confirm that the proposed method is effective in mitigating sub-synchronous oscillations.