4.2.1.4 Absorption of Moisture by Insulating Materials
Solid insulation (paper insulation) in transformers is very porous and thirstily
absorbs water. Some of the water that is dissolved in the oil is absorbed from
the oil by the cellouse (paper) winding insulation. As more water is dissolved
in the oil, more water is absorbed by the insulation of the transformer windings.
Once absorbed, it is diffi cult to remove. The most effective method for
drying out the insulation in transformers is with heat and vacuum. Sometimes
a vacuum cannot be applied in the fi eld; then the transformer insulation must
be dried by circulation of hot, dry oil. This oil should then be cooled and dried.
Since the dielectric strength of insulation is reduced by absorption of moisture,
it is important that the insulation not be allowed to absorb it in the fi rst place.