Moisture in Transformers - Part 1

There are several methods for dehydrating oil in a transformer while simultaneously attempting to dry the paper insulation. However, drying the insulation (the most important step) is much more difficult than just dehydrating the oil. Consider the following important facts with respect to water and transformers:

1. The moisture content of the oil is only a partial indicator of the amount of moisture in a transformer, because paper insulation has 100 times or more affinity for moisture compared to the oil.
2. As the oil temperature rises, some of the moisture will migrate out of the paper and into the oil.
3. In a truly wet transformer, where the paper insulation is also wet, drying the oil with a short-term dehydration procedure will NOT dry the insulation. Unless the moisture was only within the top surface of the paper insulation, within weeks of an “oil-only drying procedure”, water will migrate from the paper back into the oil at unacceptable levels.

Note the following example to demonstrate paper insulation’s high affinity for moisture:

If a transformer’s paper insulation contains 2% moisture by dry weight, then a reduction to 1% may be desirable. Depending on several factors, on average there would be about 11,700 lbs of paper/solid insulation in a 10,000 gallon transformer. This is a combination of the paper wraps on the conductor, the wood structure, and the various cardboard and pressboard components that make up a typical “insulation package”. Again, this percentage depends on the transformer design and serves as an average. Shell-form transformers generally have a larger amount of insulation.

If the 1% desired moisture removal is multiplied by the weight of paper, we calculate:
11,700 multiplied by 0.01 = 117 lbs of water to remove
+ 117 lbs of water remain in the insulation
= 234 lbs of water in the transformer

Although the concentration of moisture in the oil will vary with temperature, a fairly wet transformer could be 30 parts per million at 42°C. (This corresponds to the 2% moisture-by-dry-weight in the paper insulation from our example.)

The 30 parts per million of water in 10,000 gallons of oil calculates as:
30 ppm divided by 1,000,000 multiplied by 10,000 gal multiplied by 7.34 lbs/gal = 2.2 lbs of water = 0.26 gal of water

As illustrated, at these levels of moisture, there are 106 times (234 lbs/2.2 lbs) more water (in amount) in the paper than in the oil. Further, when considering the concentration of moisture in the paper compared to the concentration in the oil, the ratio in this case (2%/0.003%) indicates that the paper insulation has a 667 times greater concentration of moisture than the oil! Moisture’s preference for paper, rather than for oil, points out that the key to drying a transformer is to dry the paper insulation.