Transformer & dielectric oils

It insulates and it cools, in that order

The oil in a transformer has two jobs: to sustain dielectric strength between windings and core, and to carry the heat of the losses out to the radiators. The first is what fails first, and almost always because of water. A few tens of ppm of moisture drops breakdown voltage dramatically, and the water usually does not come from outside but from the ageing insulating paper, which releases water as the cellulose breaks down. This is why the oil's condition is, indirectly, a measure of the solid insulation — the part of a transformer that cannot be replaced. The second consequence is that dissolved gas analysis (DGA) is worth more than any other sample: each type of internal fault produces a characteristic gas mixture, and the oil keeps it.

Specifications and types

The base reference is IEC 60296, which sets the requirements for unused mineral insulating oil for transformers and switchgear, distinguishing inhibited grades (with added antioxidant, longer life) from uninhibited ones. For oil in service IEC 60422 applies, giving limit values for equipment in operation, and for interpreting dissolved gases, IEC 60599. The American equivalent is ASTM D3487. Breakdown voltage is measured to IEC 60156 and water content by the Karl Fischer method. Where fire risk or environmental requirements demand it — transformers inside buildings, near water or in protected areas — the alternative is ester fluids, natural or synthetic, with a far higher fire point and good biodegradability.

Handling and monitoring

Dielectric oil is the one product in the catalogue where handling matters as much as the product itself:

• Do not open the drum except to fill. Dielectric oil absorbs moisture from the air, and a few hours of exposure can take a sample out of specification.
• Filter and dry before filling, even with new oil. Breakdown voltage at delivery is not guaranteed after transport and storage.
• Schedule DGA regularly, and keep the history. One sample says little; the gas trend says almost everything.
• Do not mix inhibited with uninhibited oil without a compatibility check, nor mineral oils with esters.
• Track acid number and interfacial tension together — they show the oil ageing before its appearance changes.