Sliding-Mode Control for DFIG Rotor- and Grid-Side Converters Under Unbalanced and Harmonically Distorted Grid Voltage

Abstract

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.

Sliding-Mode Control for a DFIG-Based Wind-Power Generation System with Series Grid-Side Converter under Unbalanced Grid Voltage Conditions

“…As it is expressed in [26] the GSC must control the active and the reactive power of the grid-side by the sliding-mode control and the DC-link voltage must be set at the reference value using a PI controller. This method is not robust against Initial transient state and parameter variations (specially changing of the capacitor of the DC link) simultaneously.…”

“…Figure 4 shows the structure of the GSC. The equations of the GSC which have been represented in [26], are expressed here as…”

“…Similar to (20) The sliding-mode control of the RSC is similar to that of [26]. Based on the sliding-mode control, the RSC is used for control of the electromagnetic torque and for the stator reactive power.…”

“…Details of the simulated 2MW DFIG system with parameters were adapted from [32] are given in Appendix A and D and practical parameters of 5kW DFIG system with experimental parameters were adapted from [26], [30] are given in Appendix B and C. In accordance with [26], the set point of the stator active power is calculated from the rotor speed-optimal power curve and the set point of the stator reactive power is achieved with respect to the required 0.95 leading power factor. The simulation results are divided into four parts in which the corresponding results are compared to [26] and [32] to assess the proposed controller and evaluate its effectiveness. Sections 7.1, 7.2 shows the desirable performance of the DFIG system accompanied by SGSC under unbalanced grid voltage conditions and section 7.3discuss the robustness of the proposed control strategy.…”

“…SMC has been recently introduced for DFIG control connected to the ideal grid voltage because of its easy implementation and robustness [21][22][23][24]. In [25][26][27], SMC has been used to cancel the fluctuation in the electromagnetic torque and output power, but the final performances of applying these control methods are still not satisfactory. It seems that it is not possible to cancel the relatively large stator and rotor current unbalances, electromagnetic torque and power pulsations simultaneously in the DFIG by only using two converters because of the negativesequence voltage.…”