Improving Transient Stability of Grid-Connected Squirrel-Cage Induction Generators by Plugging Mode Operation

The majority of the fixed-speed wind turbines are equipped with squirrel-cage induction generators (IG) and ac-connected to the power system. During a fault in power system, voltage and rotor speed instability may occur, especially when the strength of the system compared to the power delivered by the IG is relatively low. This paper investigates a new method for improving the transient stability of power systems equipped with squirrel cage IGs. For stability improvement, the unique property of the induction machines which is the possibility of reversing the direction of the rotating flux is employed. In this method, after clearing the fault, operating mode of the induction machine is changed from the generating to the plugging mode for a short time. This leads to opposition between the rotating flux and mechanical torques, thus preventing the generator from further acceleration. Simulation results show that the proposed method can be effective in enhancing the transient stability. Since in this method, there is no need for accessory equipment, the proposed method is more attractive than the previous ones, from economic point of view.