In normal service life of an oil filled equipment, the insulating oil inside the equipment is subjected to deterioration, due to various causes. In normal service span of the equipment there are many maintenance occasions, when due to opening of blanking or any other reason the oil comes in contact with air. The air can also be entered in the oil due to unattended leakages in the body of the equipment.
Contact of atmospheric air with the oil causes unwanted oxidation reaction in the oil. These oxidation reactions are further accelerated due to temperature and presence of catalysts like solid copper, iron and other dissolved metallic materials in the oil.
Temperature of oil rises due to loading of the equipment and any flash or arc formed in oil filled space of the equipment. As a result of these oxidation reactions the color of the oil becomes darker and resistivity of the insulating oil decreases. At the same time acidity of the oil increases. Also the tan delta or dielectric dissipation factor of the oil increases. That means over all insulating properties of the oil deteriorated. Deteriorated oil also affects the other insulating parts (mainly paper insulation) of the equipment. Deteriorated oil decreases the normal life span of the oil immersed equipment and may increase the no load (fixed) losses of the equipment.