الفهرس 
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Abstract
Power electronics and advanced control technologies have made it possible to mitigate effects of power quality problems and maintain the operation of critical and sensitive loads. Among power quality disturbances, voltage sags, swells, and harmonics are some of the annoying problems to the sensitive industrial loads. Numerous case studies explain the importance of correcting voltage sags, which represent 70% of power quality disturbances, to avoid possibility of shutting down large production lines and consequently, the huge economical losses. The Dynamic voltage Restorer (DVR) is one of the most effective devices used to protect those loads against voltage sags disturbances. In this thesis, three different control schemes are introduced for voltage sag mitigation using DVR. The first control is an open loop control; the second is an open loop plus passive low pass filter compensation algorithm, and finally, a closed loop control scheme with injected voltage negative feedback loop. A fast technique to detect and separate the positive and negative sequence components in Synchronously Rotating Frame (SRF) is introduced for all voltages and currents components used in the schemes. It is proven that the three phase load voltages, upon balanced sags and swells, are restored to nominal (i.e. pre-sag) magnitude values in the three schemes. On unbalanced sags, the open loop with low pass filter compensation gives satisfactory results to the extent of average tolerant between voltage magnitudes of three phases of 20%. This control schemes are extensively tried and investigated as a preliminary step to the final closed loop control. The closed loop system gives promising responses of restoring nominal conditions in balanced, unbalanced, and swells of source voltages with average error of not more than 5% of nominal values. Detection and separation of positive and negative sequence components for injected, load, and source voltages, are employed and prove liability and also minimize calculations. All control schemes are developed, analyzed, and simulated by Matlab / Simulink package V7.0.
Responses to balanced and unbalanced voltage sags and swells are investigated and verified.