Performance of the Doubly-fed Induction Generator During a Voltage Sag

Due to the large increase in the installed capacity of the wind power systems, it is necessary that the wind power generation remains in operation in case of grid disturbances, since the disconnection may cause a deficit in the power generation and, thus, contribute to the destabilization of the system. For this reason, grid operators invariably require wind farms to resist to voltage sags for a specified period of time. This paper presents a theoretical and simulation analysis of the dynamic performance of the doubly-fed induction generator signals behavior in case of symmetrical or asymmetrical voltage sag in the point of common coupling and performs an analytical assessment about the back-electromotive force during normal and fault operation. It is demonstrated that voltage fluctuations and, in general, any electromagnetic disturbances can cause peaks of considerable magnitude in the generator currents and voltages, which feature heavily oscillatory behavior with grid-related frequencies. Realtime digital simulation results are presented in order to verify the generator variables, highlighting the intrinsic aspects in case of faults.