The paper presents a decoupled direct power-controlled (DPC) of a doubly fed induction generator (DFIG) connected to an unbalanced power grid, executed in an orthogonal stationary reference frame related to the stator side. The control allows to maintain constant electromagnetic torque despite stator connection to the unbalanced power grid and to achieve controllable low-voltage ride through (LVRT) capability. The control does not require any signal decomposition into positive and negative sequence order, which allows to reduce its complexity and computational requirements. Then, the paper presents positive influence of rotor current feedback in the stator-controlled reference frame on control performance. It facilitates significant transient reduction and minor steady state control improvement. All the mentioned functionalities were validated both in simulation and in laboratory conditions and the obtained results are described in the paper.

The paper presents a new type of rotor position observer using cross and dot products of the estimated and measured rotor current vector. A novel synchronization error function contains modified function atan2 and the vector products. The modification of error function is aimed at making the function monotonic, what provides faster response of the observer. Two rotor speed and position observers with the modified error function are presented. The former is based on the flux model of doubly fed induction machine DFIM and can be made in both αβ and dq frames, whereas the latter is based on the voltage steady state model of DFIM in dq frame.

The paper deals with the methods of either stator current or rotor current vector control of doubly fed induction machine connected to the unbalanced power network. Several alternative control targets, such as fixed electromagnetic torque, symmetrical stator current, or symmetrical rotor current, have been achieved. In spite of the selected target, the stator current is sinusoidal, however in some cases, it is unbalanced. The proposed vector control methods do not use separate negative sequence calculation and control. In some targets, filtration of signals is required in order to achieve symmetrical reference variables (positive sequence), which can be either stator or rotor current depending on the assumed target. Laboratory tests results have been presented in the steady state, as well as in transients. Short comparison of stator and rotor current controllers has been made on the basis of the laboratory test results.

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.

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 proportional-resonant 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.