Improving Transient Stability of Grid-Forming DFIG Based on Enhanced Hybrid Synchronization Control

Grid-forming (GFM) technique is attractive for doubly fed induction generator (DFIG)-based wind turbines (WTs) to realize its stable operation and grid-support ability under weak ac grid. Meanwhile, GFM-DFIG suffers from instability under grid voltage sag due to its poor ability to resist disturbed grid voltage. In this article, focusing on its transient stability under symmetrical grid voltage fault, the large-signal model of GFM-DFIG with voltage-orientation control (VOC) considered is established, based on which its transient behavior is studied. It is found that two types of transient instability exist during a symmetrical grid voltage fault. The first is induced by the inexistence of the postfault equilibrium point (EP). The second is induced by the excessive dynamic deviation between synchronous angle and point of common coupling (PCC) voltage angle. Given that the postfault EP and transient damping are vital for transient stability, an enhanced hybrid-synchronization control (EHSC) strategy based on adaptive Integrator is proposed, through which both the postfault EP and the transient damping can be improved. Finally, theoretical findings and effectiveness of proposed strategy are verified with a 5 kW DFIG-based experimental platform.