Controllability Enhancement of Doubly Fed Induction Generators by Use of Tapped Stator Windings

The stator voltages of Doubly Fed Induction Generators (DFIGs) are not controlled, as they are supplied from the grid, while the rotor voltages are controlled by a partially rated power converter. Modal analysis shows that the DFIGs' poorly damped modes with a natural frequency near the grid frequency cannot be controlled by the rotor voltages. Controlling both the rotor and stator voltages would allow enhancing the controllability of these DFIGs, which, however, introduces additional complexity and cost as a fully rated converter would be required on the stator side. To enhance the controllability of DFIGs, we propose the use of tapped stator windings, where only the low-voltage tapping is controlled, requiring only a small additional converter that could share the DC bus with the rotor converter. We proof the effectiveness of this proposed method by analysis of the eigenvalues of the controllability Gramian matrix for different model parameters, which is also supported by time-domain simulations.