Why Is There Moisture in Transformer Insulation?

Intuitively, the moisture in the transformer is undesirable. The goal of this article is to provide a very practical and experience-based overview to solve several common problems, answers, and opinions from the transformer manufacturer's industry and experience with having dried more than 500 transformers with moisture issues.

What are the most common problems?

What are the different humidity levels to be aware of? How to evaluate the slight, moderate, or extreme humidity of a transformer?

Is the moisture shown in the oil a reliable indicator of the condition of the solid insulation?

How does moisture affect aging and reliability?

What can be said about the economic impact of humidity on transformers and the cost of restoring reliability?

A new dry transformer with under 0.5% moisture in the insulation is the norm. How much total moisture is in the paper insulation and how much is in the oil? Only about 1% of water is present in the oil, or even less. An interesting approach is to consider a 1000 gallon transformer with an estimated 1,100 pounds of porous/paper insulation. If half of this weight is water, if it is completely in oil, it will only have a little over 5 pounds of water dissolved in 1,000 gallons (7,500 pounds) of oil. This means that a completely dry transformer oil contains 780 ppm of water! But we can see that the situation is the opposite because it depends on the temperature, usually below 10 ppm, but in a dry-type transformer, the typically expected moisture content of water does not exceed 10 to 25 ppm.

What about a transformer starting to show signs of moisture issues? How wet is too wet? First of all, how does it get wet? There are three basic ways of transformer damp:

1. Somehow, this machine was shipped from the factory and it was improperly dried. This is not so common. The factory test includes the insulation power factor test before shipment, and a value of about 0.5% indicates dryness. The lower basic insulation limit (BIL) can withstand 1%, and it is reported that some power distribution devices allow 2%. Generally speaking, the lower-level insulation power factor value is in good agreement with the dry weight percentage (%MBDW) in solid insulation

2. The next reason for transformer damp is the aging of oil and paper insulation because moisture is a by-product of the chemical reaction between oil and paper and oxygen, heat, and even trace moisture. Copper is also a catalyst for the oxidation of petroleum, which makes the control of moisture, heat, and oxygen more ideal. If there are no rising acid levels, no signs of furan (a chemical by-product of paper insulation cracking), and good D1816 dielectric strength, this cause is unlikely to happen.

3. Finally, and most commonly, moisture enters the transformer through the entry. The oil level in the transformer will vary with the load and the environment. With these changes, the oil volume may vary by 4-5% per day. If a device is not properly sealed, the oil preservation system will allow moisture to enter when the oil cools and reduces its volume. Once in, it will dissolve and will not migrate backward.

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