A power transformer in a sub-station is not only one of the costliest equipment but also one of the most important links of the power system. If the power transformer is required to give a trouble free service it should receive proper attention for its maintenance. One of the most important factor responsible for the performance of the transformer is the quality of the oil. Normally insulating oil is subjected to dielectric and moisture contents at site for the condition of the oil.

CONDITIONING OF OIL
The oil besides acting as a dielectric barrier also carries the heat away. On the other hand it absorbs moisture and gasses from the atmosphere and solid insulation aging products during the operation, resulting in considerable decrease in its break dow voltage, insulation resistance factor and increase in dielectric loss factor. Even commercial is seldom sufficiently pure for direct use in high voltage equipment, being subject to contamination in transport containers and liable to absorb unacceptab quantity of moisture and gasses because of contact with atmosphere. The conditioning process eliminates thefollowing contaminants from the oil.