Moisture in Transformers - Part 2

Moisture in Transformers - Part 2

Moisture may exist in a transformer in several forms:

1. Water vapor in the gas space of the transformer
2. Water dissolved in the oil
3. Free water suspended as droplets in the oil
4. Emulsified water contained in the decay products of oil oxidation
5. Water absorbed into and adsorbed onto the solid insulation
6. Free water that has settled to the bottom of the transformer

Paper has more affinity for moisture than oil does. For example, a typical transformer may have over 100 times more water in the paper than in the oil (by weight). Once wet, moisture removal is the corrective action that should follow. However, before moisture removal, it is important to first evaluate how the transformer became wet.

Moisture may enter a transformer in several ways:

1. Accidental Leaks. Leaking or missing gaskets for bushings, covers, pressure relief devices, lids, and gauges are a major source of moisture ingress. As temperatures vary, moisture infiltrates the leak source. Leaks at welds and pumps are also significant sources. Often a "soiling" appearance is noticeable at the site of the leak.
2. Intake of Humid Air. Free breathers without desiccants or breathers where the desiccants are exhausted are major sources that allow moisture entry from the air.
3. Contaminated Nitrogen for Gas Blankets and Nitrogen Systems. Not all nitrogen meets the required levels of dryness for effective use in electrical systems, and wet nitrogen contributes to moisture.
4. Aging of the Solid Insulation. The solid insulation is comprised of paper. Water is chemically bound to the cellulose molecules in the paper. As the paper ages, the moisture may be released.
5. Aging of the Oil. The oil also has moisture that is chemically bound to the hydrocarbon molecules. This moisture may be released as the oil oxidizes and ages.
6. Incomplete Drying at Factory or Installation. Moisture may remain in the transformer if drying is incomplete at the factory or if the oil is improperly filled during installation. The core and coil assembly and all solid insulation should be factory dried to an acceptable level. Dry oil should be used for filling during installation. Failure to properly accomplish these two tasks will result in a unit that starts out wet.

If all goes according to industry standards and good practices, a transformer will have very little moisture in the oil and in the solid insulation. It then takes a number of years of normal aging, with minimal maintenance, to reach the point where there is enough moisture present to justify a specific maintenance procedure to remove moisture to acceptable levels. Operation issues, inadequate maintenance, and not meeting minimum standards, can all cause an increase in moisture levels, requiring more frequent moisture reduction processing.