الفهرس 
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Abstract
Without a sherd doubt, power transformers are the main equipment within power transmission as well as distribution networks since they serve as a vital link in a long chain of other equipment that provides electrical energy to customers. Accordingly, the reliability of such networks is contingent on the transformer’s condition. In other words, it is very important to ensure the optimum operation of power transformers for supplying electricity to utilities efficiently. In fact, the main component inside a power transformer is its insulating system which is composed of two parts, namely the paper insulation and the insulating oil. Unfortunately, this insulating system is frequently exposed to abnormal conditions due to various causes and each cause has its own impact on the transformer’s insulating system. Therefore, the condition monitoring of a power transformer’s insulating system has massive importance according to the electrical utilities. This is due to the huge information the insulation’s state provides about the troubles that already exist within the transformer. Thus, as early as these problems are well detected, the transformer’s probability of being suddenly brought out of service decreases in a significant way. So, there are several diagnostic techniques by which the transformer’s condition can be evaluated.
Most of these techniques rely on the evaluation of the oil insulation quality. However, traditional methods of evaluation require equipment of high cost, experienced operators, and a long time for testing. Recently, many researchers have utilized optical spectroscopy techniques towards effective monitoring of the power transformer state based on oil quality analysis. Such techniques are non-destructive; in addition, they are easy to perform. In this thesis, optical spectroscopy techniques are introduced for effective condition monitoring of transformer oil. The techniques utilized are the Fourier Transform Infrared (FTIR) spectroscopy and the Ultraviolet-Visible (UV-Vis) spectroscopy. They are applied on faulted oil samples as well as thermally aged oil samples, which are prepared in the laboratory. Additionally, these samples are also characterized via conventional diagnostic techniques such as the Dissolved Gas Analysis (DGA), Dielectric Dissipation Factor (DDF) test, water content test, and Total Acid Number (TAN) test.
The main outcomes of this thesis can be summarized in the following four points: first, a methodology has been proposed for power transformers’ inception fault diagnosis depending on the information that can be extracted from the FTIR spectrum of the faulty oil insulation. This technique has proven its effectiveness and superiority over the well-known traditional diagnostic techniques such as DGA. Secondly, standard diagnostic limits have been established for the FTIR spectroscopy technique, based on a correlation made between the FTIR and DGA results of several faulted oil samples (Good linear correlation coefficient of about 90 %), for encouraging the electrical utilities utilizing this effective technique as an alternative to DGA in fault diagnosis. Thirdly, an approach has been introduced to calculate the health index percentage (HI%) of the power transformer’s oil insulation depending on the information that can be extracted from the oil’s UV-Vis spectrum. Indeed, this condition-monitoring approach has proven its efficiency. Finally, the physical mechanism behind the influence of water content, usually generated within the oil insulation due to oxidative thermal ageing, on the oil’s UV-Vis spectrum as well as the DDF measurements has been explained in more detail.