Nonlinear auto disturbance rejection control for DFIG-based wind turbines under unbalanced grid voltage circumstances

During grid voltage imbalance, doubly-fed induction generator (DFIG) wind turbines experience undesirable torque and active power oscillations due to distorted rotor fields. Consequently, these oscillations increase mechanical stress on the system and degrade its performance. To address this challenge, a simplified nonlinear active disturbance rejection control (SNADRC) strategy is proposed to regulate the negative sequence components of both rotor- and grid-side currents. Specifically, the SNADRC automatically estimates and compensates for internal system dynamics and external disturbances, thereby enhancing robustness and control performance. In addition, a novel tuning method is introduced for selecting the controller parameters to achieve fast dynamic response and minimal overshoot. The proposed control approach is experimentally validated using a 3-kW laboratory-scale DFIG setup. The results demonstrate that electromagnetic torque oscillations are reduced from 6 Nm to nearly zero, generator speed oscillations are suppressed from 7 rpm to 4 rpm, and the 120 Hz components of stator and grid active power are significantly attenuated. These findings confirm the controller's effectiveness and practical applicability in enhancing wind turbine performance under unbalanced grid conditions.