الفهرس 
Chapter 1 IntroductionOnly 14 pages are availabe for public view
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Abstract
Renewable power generation has proliferated in the last few decades, and wind energy is one of the most popular technologies in this field. Among the different wind turbine technologies, doubly-fed induction generator (DFIG)-based units are more common due to their advantages. The outstanding feature of the DFIG is the utilization of a small converter which leads to a lower cost compared to other variable speed structures with the capability of active and reactive power control. Over the last 15 years, variable speed wind turbines (WTs) with DFIGs have become the most applied WT technology and the drive train choice for up to 60% of large, >1.5 MW WTs. The typical configuration of these WTs consists of a wound rotor induction generator (WRIG) with stator winding connected directly to the grid, whereas the rotor winding is connected via slip-rings to a partially rated back-to-back converter and operating as a DFIG. In this system, the variable speed range is approximately ± 25% of the synchronous speed. The rating of the power electronic converter is only 30% of the generator capacity, which makes this concept attractive and popular from an economic point of view. Undetected generator faults may have a catastrophic effect on the turbine resulting in costly and lengthy repairs. It is desirable that faults are detected early to allow planned preventative maintenance and increased availability. A literature survey shows that bearing faults and stator insulation breakdown causes the majority of machine failures. Induction-machine stator windings insulation degradation is one of the major (about 40%) causes of machine failure. Stator faults begin with degradation of the insulation between turns, and consequently, an interturn short circuit occurs. Inter-turn short-circuits fault (ITSCF) are the most important faults in the stator winding of DFIGs which are caused by various factors such as thermal overload, electromechanical force-induced vibration, and high dv/dt voltage surges caused by adjustablespeed drives. In this thesis, a vector-based dynamic model for DFIGs will be presented which is employed with a complete variable speed wind turbine model driven by a closed-loop fourquadrant control system to investigate the effect of stator internal fault on the rotor circuit. Under healthy operation, the complete model was tested under both steady-state operation and transient operation to validate the effectiveness of the whole system. Based on Traditional Fourier Transform (TFT), an online wavelet analysis for the rotor current under faulty operation will be presented in order to identify a fault-related pattern that may be used as a reliable fault index even under different fault levels and different wind speeds. The main advantage of this methodology is that no external sensors are required as the already existingbuilt-in sensors located in the back-to-back converter can be employed for this purpose. Also, a quantitative method will be presented based on the stator reactive power analysis to provide a fault severity calculation which will be used as a trigger to initiate a Fault Ride Through (FRT) algorithm. This FRT approach will enable the machine to safely continue working even under a stator internal fault with complete prevention of power interruption or catastrophic damage that may occur to the machine windings.