Transformer oil (also referred to as insulation oil) is contained in oil tanks of power transformers and reactors, oil-filled switchgear, instrument transformers and taps. The oil in transformers is used for cooling and insulation, as the name implies. Transformer substations use various types and brands of oil. Oils of different types can be quite different in terms of dielectric strength, therefore every oil is designed for use under specific voltage.

Transformer oils may be divided into the following subgroups:

  1. Fresh new oil (without additives or stabilizers), the way it comes from the manufacturer;
  2. Regenerated (reclaimed) transformer oil;
  3. Clean dry oil (fresh new and regenerated transformer oil or a mix of the above after drying);
  4. Operational (parameters of such oil are within the required specifications for oil from the moment of operation start to regeneration);
  5. Waste oil, which has lost its qualities and does not comply with the requirements of operational oil.


There are several physical and chemical parameters of transformer oils.

Dielectric strength is one of the most important oil parameters; it is measured as breakthrough voltage. Breakthrough voltage of fresh oil must be at least 30 kV. Reduction of breakthrough voltage is a sign of contamination of the oil with water, air, fibers and other contaminants.

Tangent delta is a measure of the dielectric qualities of the oil. Dielectric losses define the quality and purity of fresh oil, or the degree of contamination of oil in operation. Increase of tangent delta (loss of dielectric qualities) leads to reduction of insulation quality of the transformer as a whole.

Most oils are light yellow. Dark color of fresh oil is a sign of low quality of the oil and incorrect production process. The color of the oil is a general measure of oil quality.

Solid particles are unsolved substances contained in the oil in the forms of sediment or suspension. These particles can include fibers, dust, products of transformer fragments dissolved in oil (such as paint, lacquer etc). Internal damage in the transformer (arcing, overheating) produces particles of ash. As the oil ages, slime forms on the insulation and decreases its dielectric performance.

Moisture content of oil should always be carefully controlled in the course of operation. Increase of moisture content is a sign of breaches in the transformer or excessive loads during operation (intensive aging of insulation under extreme temperatures).

Flashpoint of the oil is a measure of its volatility. When the oil is used in the transformer, its flashpoint gradually increases as volatile fractions evaporate. Normally, the flashpoint varies within 130 – 150oC (90 - 115oC for arctic oils) and depends on saturated vapor tension. As far as fire safety is concerned, self-ignition temperature (the temperature at which the oil in contact with air ignites without contact with open flame) is of utmost importance. This temperature is normally 350 – 400oC for transformer oils.

Acid number of the oil is the amount of potassium hydroxide (KOH) expressed in milligrams, required to neutralize free acids in 1 gram of oil. This is a measure of oil aging (formation of acidic substances in oil). Acid number must not exceed 0.25 mg KOH per 1 gram of oil.

Water soluble acids and bases, contained in the oil, indicate its poor quality. They may form as a result of incorrect production process, or when the oil is operated and the oil oxidizes. Such acids cause metal corrosion and accelerate insulation aging.

Stability is checked during operation by artificially aging batches of fresh oil in special systems. Stability of the oil indicates its longevity, i.e. life time, and is defined by two parameters: the percentage of sediment and acid number.

Setting point is a factor for oils in northern regions. This temperature is the highest temperature at which the oil in a beaker inclined 45° does not change its level within 1 minute. Unacceptable increase of oil viscosity caused by low temperature, in turn, causes damage to movable parts of the transformer (oil pumps and load regulators) and reduces heat exchange, leading to overheating and aging of insulation (especially the coils) of power-conducting parts of the transformer.

Oil gas content must remain within required limits in air-tight transformers. Total gas content is measured with chromatographs. Indirectly, this parameter is used to determine tightness of the transformer. Increased gas content (including air) in the oil degrades its performance specifications: intensifies oxidation by air oxygen and somewhat lowers dielectric strength of transformer core insulation.

Density is used to calculate weight of the delivered oil. It also indicates the content of aromatic hydrocarbons, i.e. the oil’s susceptibility to additives, hygroscopic property, resistance to the effects of the electric field etc.

Viscosity indicates oil mobility with temperature fluctuations in the transformer. Higher viscosity disrupts heat exchange, accelerates insulation aging, and also increases resistance to movable parts of the transformer (e.g. regulators).

Refractive index is an indication of naphteno-aromatic hydrocarbons content in the oil.

Blending of transformer oils.
Studies show that blending of oils in any proportion has no negative effect on the blend’s performance, i.e. no new chemical or intermolecular compounds are created. In practice, a check of the future blend’s tangent delta is required before actually blending the total volume to make sure that this parameter does not exceed regulated limits. Blending of fresh and operational oils in power transformers over 110 kV is not allowed if tangent delta of the blend is above tangent delta of either component.

As a rule, new or overhauled transformers are filled with fresh or regenerated (reclaimed) transformer oil. Prior to filling, the oil is often purified, dehydrated and degassed.

Removal of suspended unsolved contaminants (water, slime, ash, fibers etc) can be performed by selling the oil, running it through centrifuge, filtration and drying. Water can be removed from the oil using CMM or ZP series systems manufactured by GlobeCore, Inc.

Purification means removal of solid particles and slime along with a certain amount of water. Purification is usually a two stage process. First, larger solid particles (visually discernable) are removed (e.g. by a GlobeCore CMM-C or CMM-F systems). Second stage involves a more thorough purification with pressure filters. However, most systems build by GlobeCore combine all required operations in one plant and show excellent versatility.