This paper presents a new control strategy for a doubly fed induction generator (DFIG) under unbalanced network voltage conditions. Coordinated control of the grid-and rotorside converters (GSC and RSC, respectively) during voltage unbalance is proposed. Under an unbalanced supply voltage, the RSC is controlled to eliminate the torque pulsation at double supply frequency. The oscillation of the stator output active power is then compensated by the active power output from the GSC, to ensure constant active power output from the overall DFIG generation system. In order to provide precise control of the positive-and negative-sequence currents of the GSC and RSC, a current control scheme consisting of a proportional integral (PI) controller and a resonant (R) compensator is presented. The PI plus R current regulator is implemented in the positive synchronous reference frame without the need to decompose the positiveand negative-sequence components. Simulations on a 1. 5-MW DFIG system and experimental tests on a 1.5-kW prototype validate the proposed strategy. Precise control of both positive-and negative-sequence currents and simultaneous elimination of torque and total active power oscillations have been achieved. Index Terms-Converter, doubly fed induction generators (DFIGs), proportional integral (PI) plus resonant (R; PI-R), voltage unbalance, wind energy. NOMENCLATURE V g and I gGrid-side converter (GSC) output voltage and current vectors. V s and V r Stator and rotor voltage vectors. I s and I r Stator and rotor current vectors. ψ s and ψ r Stator and rotor flux linkage vectors. ω s , ω r , and ω slip Stator, rotor, and slip angular frequencies. P s and Q s Stator output active and reactive powers. P g and Q g GSC output active and reactive powers. P total Total output active from the doubly fed induction generator (DFIG) system.

This paper presents a new direct active and reactive power control (DPC) of grid-connected doubly fed induction generator (DFIG)-based wind turbine systems. The proposed DPC strategy employs a nonlinear sliding-mode control scheme to directly calculate the required rotor control voltage so as to eliminate the instantaneous errors of active and reactive powers without involving any synchronous coordinate transformations. Thus, no extra current control loops are required, thereby simplifying the system design and enhancing the transient performance. Constant converter switching frequency is achieved by using space vector modulation, which eases the designs of the power converter and the ac harmonic filter. Simulation results on a 2-MW grid-connected DFIG system are provided and compared with those of classic voltage-oriented vector control (VC) and conventional lookup table (LUT) DPC. The proposed DPC provides enhanced transient performance similar to the LUT DPC and keeps the steady-state harmonic spectra at the same level as the VC strategy. Index Terms-Constant switching frequency, direct power control (DPC), doubly fed induction generators (DFIGs), sliding-mode control (SMC), wind power. NOMENCLATURE I s , I r Stator, rotor current vectors. L m Mutual inductance. L s , L rStator, rotor self-inductances. L σ s , L σ r Stator, rotor leakage inductances. P s , Q s Stator output active and reactive powers. R s , R r Stator, rotor resistances. U s , U r Stator, rotor voltage vectors. θ r Rotor angle. ψ s , ψ r Stator, rotor flux linkage vectors. ω 1 , ω r , ω slip Stator, rotor, and slip angular frequencies. Subscripts α s , β sStationary α s β s axis. α r , β r Rotor α r β r axis.

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