Water in Transformer Oil

Water in Transformer oil is a serious issue which can reduce the insulating properties of the transformer oil.  Water also causes degradation of the paper based insulation.  This seriously and permanently damages the insulation of the winding.

Water enters the transformer through three ways

Through the Silica gel breather
If the silica gel is not monitored adequately, dry silica gel is always blue.  The silica gel should be replaced when it changes color.  If the silica gel is saturated, it may no longer filter the air and water can enter the transformer in the form of moisture.

Through improper sealing of accessories
Leaking gaskets, loose fitting components, cracked bushings can cause rain water to enter the transformer.

Water in Transformer oil exists in two forms as free water and as water in solution with the oil.  Pure oil and water do not mix.  However, over a period of time due to contamination and degradation the oil absorbs some water.

Disintegration of Cellulose
The insulation of the transformer winding is made of cellulose based paper.  At high temperatures, cellulose disintegrates and one of the byproducts is water.

Sampling the oil for Water content
The oil should be sampled for water content periodically.  It should be ensured that the sample collected is representative of the entire oil.  For instance, free water usually is at the bottom of the transformer tank.  Thus oil collected from the bottom valve may indicate a higher value of water content.  Thus samples should be collected from the upper and the lower valves of the transformer.

If the water content is found to be excessive.  The oil needs to be drained and dehydrated.