An Analytical method for detecting instability in grid-connected doubly-fed induction generator

Doubly-fed induction generator (DFIG) is among the most attractive technologies in wind farms. As the penetration of wind power into the grid has increased in recent years, protecting of wind farm's generators against grid disturbances has become a major issue for the power system industry. Transient instability (TI) is among the most frequent phenomena due to severe grid disturbances. This phenomenon might cause significant damages to network equipment. In DFIG based wind turbine (WT), when the crowbar circuit is activated, the rotor winding is short-circuited and the power converter is blocked. At this moment, the DFIG operational mode is temporarily changed to squirrel-cage induction generator (SCIG). This lead to reactive power absorption and voltage drop, that might cause electromechanical oscillations and unstable condition. This study presents an analytical method for detection TI of a grid-connected DFIG based on analysis of angular velocity and acceleration data, which are readily obtained from locally measured quantities. This technique works on a setting-free manner, i.e. revising would not be required even when the network configuration changes, which is a favourable feature in protective relaying. The performance of the proposed method is verified using MATLAB/Simulink. Simulation results prove the method's reliability and simplicity.