What to do if transformer has an excessive moisture content

Moisture measurement

The term “moisture” in the transformer industry is commonly used to indicate water which is absorbed in the paper or dissolved in the oil. Occasionally, the terms “water” or “water content” are used as an alternative way to describe the same substance.

Water in transformers can be found in different parts of the insulation system. It can accumulate in solid insulation, be dissolved in oil, or be found in the form of liquid water at the core or bottom of a transformer.

If it has been determined that a transformer has an excessive moisture content, either through water-in-oil analysis or dew-point analysis, the transformer should be dried out. Otherwise, the presence of moisture in the kraft paper will definitely shorten the life due to chemical decomposition through oxidation of the cellulose.

Simple hot oil flush

A simple hot oil flush is one method, although it is fairly slow. Oil is pumped through a filter press, which contains paper filter elements, and then heated and sprayed onto the core and coils. The hot oil has a greater affinity for moisture than cold oil and will pick up some moisture from the core and coil insulation.

Running the oil through a filter press allows the filter elements to remove some of the moisture in the oil. Cooling the oil before it goes through the filter press will enhance the removal of moisture from the oil.

To spray hot oil

A better method is to spray hot oil into the transformer under vacuum. The combination of the heat from the hot oil and the vacuum will cause the moisture in the insulation to ‘‘flash off’’ and dry the transformer. A cold trap can enhance the action of a mechanical vacuum pump. Once the mechanical pump removes all the air from the transformer tank, the only gas left in this space is H₂O.

A cold trap using a dry ice and acetone jacket will instantly freeze any water vapor with the low-temperature ice having a very low vapor pressure.

The cold trap thus acts like an auxiliary vacuum pump. The ice provides a direct measurement of the amount of moisture removed from the cellulose.

Online Power Transformer Moisture Removal System (VIDEO)

Moisture in transformers (VIDEO)

The research conducted at Monash University has shown that, for the accurate moisture assessment of transformer insulation, the continuous monitoring of a number of parameters, including transformer load, temperature and oil relative saturation, is required. The measurement of water solubility characteristic of oil in the transformer is also required.

How is it possible to identify a transformer with moisture concerns for further monitoring? There is a demand for a classification procedure, which would rank transformers by moisture levels.

References //

• Power Transformers Principles and Applications – John J. Winders, Jr. (Amazon)
• Moisture assessment in power transformers – V. G. Davydov, O. Roizman