Peculiarities of Transformer Oil Servicing

Transformer oil is an petroleum product, used to insolate the active parts of transformers under voltage as well as cool and transfer heat during operation.

The performance characteristics of this oil product depend on its chemical composition that is determined from the raw material and methods uesed in its purification.  The commissioning of new electric power transformers requires only fresh oil that has not been previously in service.  Each batch of the product intended for transformer refilling should be certified by the supplier.  But, even fresh new oil delivered to the power facilities should be tested and, if necessary, cleaned of mechanical impurities, moisture and gases before being placed into the transformer.

Performance characteristics of transformer oil are evaluated by the following indicators:

  • dielectric strength;
  • loss tangent;
  • moisture content;
  • gas content; and
  • the presence of mechanical impurities.

Centrifuge processing of transformer oil refers to the classical methods.  Processing by centrifuge forces removes moisture and suspended mechanical particles from the oil.  Centrifugal treatment is applied when refilling power transformers with voltage up to 35 kV or used as a method of pre-treatment.  Long centrifugation is undesirable since it can lead to removal of antioxidant additives that can lead to the premature oxidation of the oil.

Transformer Oil Filtration

Filtration consists of transformer oil being passed through special porous partitions that retain unwanted impurities.

Absorption Treatment of Transformer Oil

The Absorption Treatment of transformer oil absorbs water and other impurities using special substances called “adsorbents.”  Adsorbents may include synthetic zeolites that intended to remove dissolved gasses and moisture from the oil.

Transformer Oil Treatment with Vacuum Units

Vacuum filtration systems provide for preheating of transformer oil up to between 50 to 60 ºС.  Then, the oil is sprayed in the first stage degasser and  flows down in a thin layer along the surface of a series of Rasching Rings.  Simultaneously, the first stage is vacummized by a vacuum pump.   Gas and moisture vapors are pumped through a zeolite cartridge and air filter.  The final drying and degassing of transformer oil is performed in the space of the second stage degasser.  Purified oil comes to the transformer or tank through a fine filter.


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