Effect of modeling of induction generator based wind generating systems on CCT

For understanding the dynamic behavior and assessing the stability of induction generator based wind generation system, the system should be modeled accurately. Power system transient stability is usually assessed in terms of critical clearing time (CCT) of fault. CCT determines the time for which the wind generating systems can be connected to the grid, when the grid is subjected to a fault. In the literature, CCT of induction generator based wind generation system connected to the grid was determined by assuming the mechanical torque output of wind turbine as constant throughout the fault simulation. However, while determining CCT, the mechanical torque output of wind turbine, before and after the fault cannot be considered as a constant as the mechanical torque-speed characteristic of a wind turbine depends on the wind speed and the slip of the induction generator connected to the wind turbine. During fault, the slip of the induction generator changes and hence mechanical torque output of wind turbine also changes accordingly. The results of CCTs obtained by considering nonlinear mechanical torque-speed characteristic of wind turbine are presented and are compared with CCTs obtained with the mechanical torque output of wind turbine assumed constant.