الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
This thesis tackles the following issues: wind power variability and voltage fluctuation, the response of wind turbines equipped with Doubly Fed Induction Generators (DFIGs) to voltage dip disturbances, and the performance of DFIG-based wind turbines at wind-range borders.
In the context of wind power variability and voltage fluctuation, the incident wind is stochastically changing; therefore, the output power and voltage of fixed-speed direct-connected wind turbines fluctuate. Comprehensive models for the wind speed, wind turbine and flickermeter are implemented to accurately track the instantaneous power and voltage variations. Grid and site-based factors affecting the terminal voltage fluctuations have been examined. An efficient and limited rated electrolyzer/fuel cell combination along with a proposed control scheme is used to control the ramp rate of the wind power and enhance the voltage quality at the Point of Common Coupling (PCC). The control scheme minimizes the losses and utilizes reduced-rated converters to interface the fuel cell and the electrolyzer.
Recent versions of grid codes demand that each wind turbine be supported by Fault-Ride Through (FRT) capability in a trial to keep the turbine operating during grid fault conditions. This work presents a new FRT strategy for wind-power DFIGs. The proposed approach depends on simultaneous switching of a crowbar set and the reconfiguration of the rotor-side converter with appropriate Transient Control Mode (TCM). The rotor-side converter is paralleled with the grid-side converter to feed reactive current to the grid during the fault. The TCM is applied to the paralleled converters to regulate the reactive current injection during the fault. Theoretical analysis and intensive simulation results show that the proposed FRT technique is a reliable and adequate solution to keep the DFIG operating during sharp voltage sags occurring outside its protection zone.
The efficiency of DFIG-based wind turbines experiences a suboptimal operation in the vicinity of both the cut-in wind speed and the turbine-rated wind speed. The degraded operation at low wind speeds is due to increased magnetization losses, dc link voltage restriction and unregulated reactive power flow. On the other hand, the reasons for the suboptimal efficiency when operating close to the turbine-rated wind speed are related to unsupervised reactive power flow and the restricted converter ratings. Reforming such deficient operations has been tackled in this thesis. A positive approach utilizing a slip extension to increase the wind power capturing capability at low wind speed operation has been proposed. An analytical derivation to determine the optimal reactive current contribution of the rotor-side converter that minimizes the total DFIG system losses is developed. The effect of the converter ratings on the DFIG performance has been discussed. Further, the modified control scheme supported by slip expansion and by an assessment unit to set the optimal reactive current contribution of the rotor-side converter has been developed. Comparative simulation results prove that the proposed modifications are positively contributing to counter the suboptimal wind-power DFIG performance at wind speed range boundaries and significantly increase the annually energy production.