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Regarding doubly fed induction generator (DFIG) operation, unbalanced and harmonically distorted grid voltage conditions have been treated as two separate control problems. This paper reports a solution for the rotor-and grid-side power converters, which allows one to keep the DFIG successfully in operation under both grid voltage conditions. The proposed solution is based on sliding-mode control (SMC). The rotor-side converter is commanded so that the electromagnetic torque and the stator reactive power remain free of fluctuations that arise during grid voltage disturbances. Meanwhile, the grid-side converter ensures both constant DC-link voltage and steady active power output from the overall system. The developed algorithms turn out being robust against parameter variations and of fast dynamic response. In addition, none of the converters need either voltage or current positive and negative sequences extraction. The simulation results presented demonstrate the appropriateness of SMC to face such disturbed scenarios. Finally, the stability proof of both converters' control algorithms is provided in the appendices.Index Terms-Doubly fed induction generators, harmonic distortion, power control, unbalanced voltage, variable structure systems, wind power generation . NOMENCLATURE C eqDC-link equivalent capacitance. e n Grid voltage's space vector. i g Grid-side converter current's space vector. i r , i s Rotor and stator currents' space vectors. L g Grid-side line inductance. L m , L r , L s Magnetizing, rotor and stator inductances. L σ r , L σ s Rotor and stator leakage inductances. P Number of pole pairs. P e Electromagnetic power. P g , Q gGrid-side converter output active and reactive powers.

Control algorithms for the rotor-and grid-side power converters of a double-fed induction generator (DFIG)-based wind turbine under non-ideal grid voltage conditions are proposed, and guidelines for tuning the controller parameters are presented. The control schemes are based on sliding-mode control (SMC) theory. Apart from directly controlling the DFIG's average active and reactive powers, the proposed methods also fulfil two additional control targets during voltage unbalance and harmonic distortion, that is, the rotor-side converter (RSC) eliminating electromagnetic torque fluctuations and the gridside converter (GSC) compensating for the stator current harmonics to ensure a sinusoidal total current from the overall generating unit. The described control strategies are proved to be robust against parameter deviations and of fast dynamic response. In spite of the discontinuous nature of the standard SMC, constant converter switching frequency is achieved. Besides, the RSC control algorithm does not require a phase-locked loop and, furthermore, there is no need for decomposing the grid voltage and different currents into symmetrical sequences or harmonic components in any of the converters' control systems. Finally, the excellent performance of the system, as well as its robustness, is verified by means of simulation results under different grid voltage conditions.

Control algorithms for both the rotor-and grid-side power converters of a wind turbine-driven doubly-fed induction generator (DFIG) are detailed, and tuning equations are also provided to assist adjustment of their gains and constants. Those algorithms are based on the second-order sliding-mode control (2-SMC) approach, and they allow the wind turbine to satisfactorily operate under grid voltage non-idealities, such as simultaneously distorted and unbalanced scenarios. The presented solution turns out to be robust against parameter deviations and disturbances, and of high-performance dynamic response. Moreover, it drives the transistors of both power converters at a constant switching frequency, also avoiding decomposition in symmetrical sequences of either the grid voltage or currents. The controllers proposed for the two power converters are validated through experimentation on a 7-kW DFIG test bench subject to a significantly unbalanced and harmonically distorted grid voltage. Their robustness in the presence of both substantial parameter mismatch and disturbances attributable to wind variability is also assessed.

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This study presents a second-order sliding-mode control (2-SMC) scheme for a wind turbine-driven doubly fed induction generator (DFIG). The tasks of grid synchronisation and power control are undertaken by two different algorithms, designed to command the rotor-side converter at a fixed switching frequency. Effective tuning equations for the parameters of both controllers are derived. A procedure is also provided that guarantees bumpless transfer between the two controllers at the instant of connecting the DFIG to the grid. The resulting 2-SMC scheme is experimentally validated on a laboratory-scale 7 kW DFIG test bench. Experimental results evidence both the high dynamic performance and the superior robustness achieved with the proposed control scheme.

Several strategies have been proposed for operating doubly-fed induction generator (DFIG)-based wind turbine under unbalanced grid voltage conditions. This study focuses on those strategies in which the rotor-side power converter aims at eliminating the oscillations affecting the electromagnetic torque and the stator reactive power. Given the limited size of the DFIG's power converters, and hence their tolerable current and voltage boundaries, this study analyzes which DFIG power generation capability is under unbalanced grid voltage, and therefrom derives rotor current and stator power controllable ranges. Besides, the feasibility regions of the DFIG are also deduced for different types of imbalance. Furthermore, and based on the outcome of previous analysis, a modified rotor current limiter, as also its equivalent stator power limiter, is proposed. In contrast to the conventional ones, the limiters proposed here take into account that imbalances may arise in the grid voltage. As a consequence, system's overall performance is considerably enhanced under unbalanced grid voltage conditions. Finally, simulation results establish the validity of the treated issues.Index Terms-Doubly-fed induction generators, feasibility region, industrial power system control, unbalanced voltage, vector control, wind power generation. NOMENCLATUREi r , i s Rotor and stator current space-vectors. I r peak Maximum permissible rotor current. L m Magnetizing inductance. L r , L s Rotor and stator inductances. L r Rotor transient inductance. P Number of pole pairs. P s , Q s Stator-side active and reactive powers. R r , R s Rotor and stator resistances. s Slip of the generator. T e Electromagnetic torque. v c Space-vector of the voltage induced in rotor windings.