Generally, it is recommended to use oils of different grades separately from each other, i. e. not mixing transformer oils together. If necessary, only mineral oils (made from naphtha) can be mixed. Before such mixing  the oils are tested  on compatibility, which includes:

  • Mixing  oils in the planned proportion, or if not known – in a 1: 1 ratio;
  • Determining the dielectric loss tangent of the mixture and the initial oils at 90 ° C;
  • Creating artificial aging conditions for the mixture and base oils in an open tank at a 95 ± 2° C for 72 hours (continuously);
  • Determining the dielectric loss tangent of the mixture and the initial oils after aging under the same conditions.

When mixing fresh and regenerated oils, the mixture is considered to have withstood the test, if its properties were not worse than the properties of each components.

Mixing of transformer oils is not permitted when the mixture is intended for use in other equipment.


If the dielectric loss tangent of the mixture at  90 ° C exceeds the highest dielectric loss tangent of the component, then it can only be used in oil switches.

Transformer oils of GK grade are added to power and measuring transformers, even if they have oils of other grades. It should be remembered that the hydrocracked GK grade oil has high performance properties and also high oxidation stability in comparison to oils of other brands, therefore it is recommended to be used without mixing. The best results  are achieved  by mixing  GK grade oils with oils brands TKP, T-750, T-1500, VG and Nytro.

If a high-voltage bushing is filled with T-750 oil,  the GK-grade oil can be used to refill, but no more than 20% of the total volume. When the refilling percentage needs to be higher for any reason, the  mixture should be tested  for compatibility.

If oil in the equipment has an acid number of 0.08 mg KOH / g or more, it is not recommended to add GK oil or similar oils with low  content of aromatic hydrocarbons.

When the oil in electrical equipment is found unsuitable by its acid number, the content of water-soluble acids, the presence of a soluble and insoluble precipitate, the dielectric loss tangent – such oil requires regeneration or replacement. Used oil is drained from the equipment with obligatory removal of residues and aging products from solid insulation.

Replacement and regeneration is associated with large losses of time and, consequently, long downtime of the equipment.


GlobeCore developed a unique technology that allows to regenerate transformer oils on energised equipment. CMM-R station connects directly to a transformer, as a result the oil circulates in a closed loop. The contaminated oil enters the station, where it recovers its properties, and clean oil flows back into the transformer, where it flushes out the hard insulation from contamination and aging products. Such system regenerates oil and solid insulation. The impurities are washed out and absorbed by a special sorbent – Fuller’s earth. This sorbent has a natural origin and can be reactivated, i.e. recover its properties. The sorbent does not require recycling, which reduces the environmental polution. GlobeCore technologies are eco-friendly, economical and always ready to solve even the most difficult tasks!

Mixing of uninhibited oils with fresh inhibited oils is not permissible without additives (for example ionol) that make such mixtures usable for power transformers.

Ionol is administered in an amount of 0.25% of the total weight of the mixture. After that, it is tested for stability against oxidation (should be above the stability of uninhibited oil).

The question of mixing transformer oils is very complex and in general does not have a universal solution algorithm. If there are difficulties, it is better to contact a specialized organization for qualified consultations.

обновлено: January 2, 2018 автором: globecore