Subsynchronous Resonance Mitigation for Series-Compensated DFIG-Based Wind Farm by Using Two-Degree-of-Freedom Control Strategy

This paper investigates a special class of dynamic power system problem, namely subsynchronous resonance (SSR) resulted from a series-compensated network connecting doubly-fed induction generator (DFIG) based wind farms. A novel two-degree-of-freedom (2DOF) control strategy combined with a damping control loop is designed and analyzed for enhancing the system stability and alleviates the SSR that may arise due to the induction generator effect (IGE). The proposed control strategy is tested at different operating conditions of series compensation levels and low wind speeds to ensure the system stability. The doubly-fed induction generator based wind farms without the proposed control strategy leads to overall system instability during high series compensation and low wind speeds. Hence, the mitigation of the SSR and damping enhancement are critical to the entire power system stability. A reliable way of analyzing the system and designing effective control strategies against SSR based on the eigenvalue analysis and impedance based stability criterion is deployed. Moreover, analytical explanations have been elaborated to verify the procedure of the controller design. Fault ride-though capability has also been investigated with the proposed control strategy that is flexible to be integrated with the FRT schemes so as to assist the wind farm in mitigating the SSR during the fault recovery stage. Finally, time domain simulations are carried out to demonstrate the effectiveness of the proposed control strategy for mitigating the SSR and damping power system oscillations.