What is transformer oil?

Transformer oil or Electrical oil is often a collective term for “insulating liquid” and need not actually be an oil.

However the most commonly used insulation liquid is a mineral based oil to BS EN 60296:2012 (unused oil) or BS 148:2009 (reclaimed oil). Other regularly used insulating liquids are MIDEL 7131, which has a higher flashpoint and is biodegradable, and silicone transformer liquid such as Dow Corning’s Xiameter.

Once in electrical equipment the oil must be meet BS EN 60422:2013 (Maintenance and supervision of oil filled equipment).


Why do transformers have insulating liquid?

The oil in electrical equipment provides three key functions:-

  • Cooling: Be it forced or natural circulation the liquid carries heat away from the core to the cooling fins or outer case. Preventing equipment from overheating is an important aspect of equipment maintenance to ensure the longest possible working life of the equipment. Laboratory research found that an increase of just 8 oC halves the working life of a transformer.
  • Insulation: Impregnated in solid insulation and circulating freely within the equipment the liquid plays the vital role of providing electrical protection and in the case of switchgear extinguishing arcs. Moisture and particles has a negative impact of the insulation properties of the liquid.
  • Information: Data from internal events such as partial discharges, heating and cellulose degradation are all stored in the oil for professional analysis and interpretation.

How is transformer oil made?

Transformer oil is a highly refined oil from crude.

The basic product is distilled and then heat treated to over 300oC to remove nitrogen, sulphur and oxygen whilst reducing aromatic content.

What if the liquid fails to meet specification?

At this stage many engineers and sub-contractors simply “default” in to replacing expensive insulating liquid. This is both technically incorrect and financially punitive.

Using mineral oil as an example as most equipment in the UK uses this medium, the British standard for moisture in category C distribution equipment (<72.5kV) is 20mg/Kg, whereas the specification of a drum of new oil is up to 40mg/Kg.

The British standard for new oil actually states “Before filling the electrical equipment, the oil should be treated to meet the requirements of IEC 60422” (BS EN 60422:2013).

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