All drying methods share a common objective, the removal of free and dissolved water from the cellulose insulation as well as the insulating oil.
There are two reasons for transformer dehydration:
• To avoid near-term failure
• To extend or maximize reliable transformer operating life
Solid insulation has a much higher affinity for moisture than oil does. Depending on temperature, paper insulation will hold from 300 to 3000 times more moisture than oil will.
The transformer with more than 4.5% water content in the solid insulation runs a very real risk of flashover across the insulation at normal and elevated operating temperatures. The moisture absorbed in the solid insulation will form conductive paths from winding to winding, to the core or to the ground.
Choosing the method of drying the insulation depends on the following factors:
• Transformer type
• Environmental conditions
• Time limitations
• Thickness of insulating parts
• Moisture degree
For transformers whose tanks are vacuum-proof, we apply the following methods:
• Cycles involving: heating, oil draining and vacuum drying of the transformer insulation
• Spraying hot oil over the windings of the transformer while the tank is under vacuum
For transformers whose tanks are not vacuum-proof, drying of the solid insulation is achieved by multiple circulations of hot oil (usually 85°C) between the transformer and the oil reconditioning equipment. To accelerate the drying process, depending on the specific conditions, a partial vacuum may be applied in the tank or dry inert gas can be pressured in.