الفهرس 
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Abstract
In recent years a great deal of attention has been focused on the supply of transformer oils and trying to find a way to overcome the problem of accumulating used transformer oils. This was the motive for this thesis which introduces some investigations on the characteristics of transformer oils under different conditions. These transformer oils have been in service for different periods of time in actual transformers operating at an industrial site (KlMA).
In this thesis mlxmg of different transformer oils has been studied and the impacts of the individual transformer oil properties, base and water content on the resultant mixture have been illustrated. A new technique to determine the allowed range of mixing ratios has been developed and introduced for the first time in this thesis.
Also, the effects of repeated breakdown tests and purification process on the physical, chemical and electrical properties of used and new transformer oils have been thoroughly investigated.
Finally, the deposits which were found in some transformers were analysed to determine their constitutions and their relation to the transformer oils and the operating conditions of the transformers. These deposits have never been analysed before. The infrared and atomic absorption spectroscopy analyses were implemented in this Shldy.