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The paper presents properties and control methods of grid-connected doubly-fed induction generator (DFIG). The paper is focused on unbalanced grid operation, because presently it is the biggest challenge for this type of generator. There are discussed three most popular methods -direct power control, direct torque control, and vector control with proportionalresonant PR controllers of rotor current in stationary alpha-beta frame. In each method, the reference signals of electromagnetic torque, instantaneous power components and rotor current components are calculated without sequence decomposition.

The paper presents a control method for the three-phase power converter operating under unbalanced grid voltage conditions. The method uses a new transformation to the non-Cartesian frame, which makes the controlled current vector components balanced in this frame even if originally the three-phase current is referenced as unbalanced. Furthermore, Park’s transformation makes the controlled variables constant, which allows to apply proportional–integral terms as current controllers independent of the required control target. Several control targets known from literature have been analyzed with regard to the required new transformation parameters, and the transformation parameters for all targets have been found. Simulation results are shown to prove the theoretical analysis, and the experimental test results are presented as practical validation of the proposed use of the non-Cartesian frame in control.

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