Direct torque control of a doubly fed induction generator working with unbalanced power grid

The paper describes direct torque control of a doubly fed induction generator (DFIG) executed in the stator stationary reference frame. A full model of DFIG is presented, and relations for the rotor voltages are derived. The case with symmetrical and asymmetrical grid voltage operation is presented along with the corresponding simulation and experimental results. As a control target, the minimization of the torque oscillations at sinusoidal stator current is chosen. No decomposition into positive and negative voltage sequence for presented algorithm is necessary. The proportional-resonance (PR) regulator tuned for fundamental harmonic of grid voltage frequency is applied. As the torque value is independent of the reference frame, the variables are not synchronized neither with the grid voltage nor with the stator flux vector. The control method is presented, and its performance for balanced and unbalanced voltage is given. To obtain the required target, the auxiliary scalar variable calculated as a dot product of stator flux and stator current is used.