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In this paper, the behavior of a doubly fed induction generator (DFIG) is studied under unbalanced grid voltage conditions. It is shown that if no special control efforts are employed, the behavior of the generator is deteriorated, basically due to two reasons: electromagnetic torque oscillations and nonsinusoidal current exchange with the grid. These phenomena are first analyzed theoretically as a function of the stator active and reactive instantaneous power exchange by the stator of the DFIG and the grid-side converter (GSC). This analysis provides the main ideas for generation of the active and reactive power references for the rotor-side converter (RSC) and the GSC, controlled by means of direct power control techniques. Therefore, this paper proposes a new algorithm that generates the RSC power references, without the necessity of a sequence component extraction, in order to eliminate torque oscillations and achieve sinusoidal stator currents exchange. On the contrary, the GSC power references are provided by means of voltage and current sequence extraction. Finally, simulation and experimental results successfully validate the proposed power reference generation methods. Index Terms-Doubly fed induction generator (DFIG), direct power control (DPC), unbalanced voltage. NOMENCLATURE L hMutual inductance. L s , L r Stator and rotor self-inductances. P, Q, S Active, reactive, and apparent powers. R s , R r Stator and rotor resistances. T em Electromagnetic torque. V busVoltage of the dc side of the voltage-source converter (VSC). Ψ, v, i Flux, voltage, and current space vectors. ω s , ω m , ω r Synchronous, rotor, and slip speeds. Superscripts and Subscripts s, r, gStator, rotor, and grid reference frame for space vectors. ). M. A. Rodríguez is with Ingeteam Transmission and Distribution S.A.,

The power generation system with a doubly fed induction generator (DFIG), which can be used as an autonomous power system after the loss of mains in a distributed generation network, is described. After the mains outage, a fixed frequency and an amplitude of the output voltage are obtained, despite the variable rotor speed. For this reason, it can be successfully applied in the variable-speed wind turbines, adjustable speed water plants, or diesel engines. Moreover, the stand-alone operation of DFIG is useful in a flywheel-based high-energy rotary uninterruptible power supply system. An output voltage is controlled directly by the synchronization of an actual voltage vector with the reference vector represented in a synchronously rotating polar frame. The rotor current angular speed is obtained as a result of vectorial phase-locked loop operation. Any sensors or estimators of the rotor speed or position are unnecessary. Both amplitude and angle control loops are linear. The use of stand-alone operation in grid-connected systems requires mains outage detection. Also, the grid voltage recovery requires a method of synchronization and soft connection of a generator to the grid. The proposed methods of output voltage control, synchronization, and detection of mains loss were tested in a laboratory system.

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High harmonic contents of the stator currents due to uncontrolled x − y subspace dramatically overshadow the performance of the switching table-based direct torque control (ST-DTC) strategy for multi-phase drives. The concept of ST-DTC based on virtual voltage vectors (VVs) has been frequently developed to alleviate this problem. However, VVs with fixed duty ratios are incapable of compensating inherent machine/converter asymmetries and dead time harmonics, which are mapped into the x − y subspace. To solve this problem, this paper develops a dynamic duty-ratio-based DTC (DDR-DTC) technique for six-phase induction machine (6PIM) drives, where the duty ratios of the selected VVs are not constant anymore. These duty ratios are updated based on the x − y voltage commands arising from the closed-loop x−y current controllers over every sampling period. The attained merits over and above the conventional fixed duty-ratio-based DTC (FDR-DTC) schemes include effective x − y current cancellation without increase in average switching frequency and decrease in DC-link utilization. Experimental results are presented to validate the effectiveness of the proposed control technique.