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.

No abstract

No abstract

No abstract

No abstract

His research interests include modeling and control of grid-integrated power converters, in particular on stability analysis and control of grid-connected renewable generations in electromagnetic timescale. Her research interests include dynamic modeling, stability analysis and control of power system with multi power converters.

This paper proposes an improved direct power control (DPC) strategy of grid-connected wind-turbine-driven doubly fed induction generators (DFIGs) when the grid voltage is unbalanced. The DPC scheme is based on the sliding mode control (SMC) approach, which directly regulates the instantaneous active and reactive powers in the stator stationary reference frame without the requirement of either synchronous coordinate transformation or phase angle tracking of grid voltage. The behavior of DFIGs by the conventional SMC-DPC, which takes no negative-sequence voltage into consideration, is analyzed under unbalanced grid voltage conditions. A novel power compensation method is proposed for the SMC-based DPC during network unbalance to achieve three selective control targets, i.e., obtaining sinusoidal and symmetrical stator current, removing stator interchanging reactive power ripples and canceling stator output active power oscillations, respectively. The active and reactive power compensation components are calculated via a simple method and the proposed three control targets can be achieved, respectively, without the need of extracting negative-sequence stator current components. Experimental results on a 2 kW DFIG prototype are presented to verify the correctness and validity of the proposed control strategy and power compensation method.Index Terms-Direct power control (DPC), doubly fed induction generator (DFIG), power compensation, sliding mode control (SMC), unbalanced grid voltage, wind turbine. NOMENCLATURE U s , V r