Understanding the Doubly Fed Induction Generator During Voltage Dips

The doubly fed induction generator (DFIG) is normally controlled with stator-flux orientation. The DFIG is a poorly damped system with a natural frequency near the grid frequency. This leads to natural oscillations on the stator flux and on other variables. When voltage dips occur, there are oscillations on the stator flux that produces electromotive forces and consequently disturbances on the rotor currents. This paper presents a comprehensive study of the DFIG during voltage dips. The method presented in this paper, valid for symmetric and for asymmetric voltage dips, is based on the classical model of the induction machine with stator flux orientation and neglects only a small part of the cross-coupling terms. The response depends on the design methodology of the proportional-integral inner controllers. Analytical, simulation, and experimental results are shown. The analysis and results can be divided into two sets depending if the voltage dip magnitude is smaller or deeper than 0.5 p.u.