Comparison of active stabilization methods for the doubly-fed induction generator - quadrature versus direct inner control loops

The doubly-fed induction generator DFIG, when controlled with the stator flux oriented method with impressed rotor currents, is characterized by oscillations on the flux and torque and an unstable region. This region occurs when direct rotor current is bigger than a critical value. Several techniques can be used for the damping and stabilization of this system. Some methods are based in additional inner closed loops for damping and stabilization. This paper presents a comparison of two methods for damping and stabilization of the DFIG. Both are based in additional inner current loops. The first method, in function of the stator flux amplitude, corrects the direct current reference adding a term proportional to the stator flux error. The second method, in function of an internal angle, acts on the quadrature impressed rotor current reference. The stability criteria for both methods are analysed and compared. It is shown that, in per unit values, the two methods are equivalent leading to equal damping for equal damping constants. Both can be used at the same time as a vector damping method.