In this paper, the dynamic performance of grid connected Wind Energy Conversion System (WECS) is analysed in terms of the newly defined concept of rotor speed stability. The WECS is considered as a fixed-speed system that is equipped with a squirrel-cage induction generator. The drive-train is represented as a two-mass model. Results show that for a particular fault simulated the voltage at the Point of Common Coupling (PCC) drops below 80% immediately after fault application and settles at a low value. The rotor speed of induction generators becomes unstable. In order to improve the low voltage ride-through of WECS under fault conditions and to damp the rotor speed oscillations of induction generator, an Unified Power Flow Controller (UPFC) is employed. The gains of this FACTS controller are tuned with a simple Genetic Algorithm (GA). It is observed that UPFC helps not only in regulating the voltage, but also in mitigating the rotor speed instability. (C) 2008 Elsevier Ltd. All rights reserved.