Stability Assessment and Enhanced Control of DFIG-Based WTs During Weak AC Grid

According to the latest grid codes, wind turbines (WTs) are required to inject certain amount of reactive and active current into the faulty grid during voltage sags. However, studies have shown that the doubly fed induction generator (DFIG)-based WTs may lose stability under weak grid condition. So the impact of grid code requirement on DFIG system stability is worthy of investigation. To address such issue, a comprehensive small-signal admittance model of DFIG system is established in this paper. Then the influence of reactive/active current settings during grid fault is analyzed in detail. Furthermore, the stable operation region is derived, considering three key factors, i.e., the current demand of grid codes, capacity of WTs and stability constraint. Besides, in order to extend the stable operation region of the DFIG system, an enhanced control strategy based on voltage disturbance compensation is put forward. Finally, simulations and experiments are performed to validate the correctness of the theoretical analysis and effectiveness of the proposed control.