Oil treatment

Transformer oil treatment

This basic information on transformer oil used in power and measuring transformers, reaction equipment and oil filled switches and the primary ways of treating this oil may be of benefit to owners of oil facilities and oil filled equipment.

Transformer oil is a high purity and low viscosity mineral oil. Its main purpose is insulation of energized parts and components of a transformer and dissipation of heat from such components which generate heat during operation, as well as keeping solid insulation dry.

In the long term operation the oil in the transformer changes (“ages”) and its performance fall to extremely low levels. The main reasons for oil aging are: hydrocarbon oxidation by oxygen from the air, electric field influence, contamination by solid partices etc.


Purifiacation of transformer oil is a preventive technique of transformer servicing to extent its life time. Transformer oil can be completely purified and restored, made as new. Due to high cost of new oil, the following methods of oil treatment are used:

  • centrifuge treatment. This technique removes water and solids from the oil by application of centrifugal force. The method only allows to remove emulsified water and such solids with specific gravity higher than that of the oil. This method is used to prepare oil for filling into power transformer up to 35 kV;
  • filtration. Transformer oil is filtered through porous screens, which capture the contaminants contained in the oil;
  • adsorption treatment. This process is based on adsorption of water and other contaminants by various adsorbents. A commonplace material used as a sorbent are various synthetic zeolites (mineral which can adsorb and release water). Zeolite treatment allows to remove solved water from the oil;
  • vacuum treatment. The main element in this case is a degasser. In this process, the transformer oil is heated to 50 – 60 degrees and passes through all degassing stages, then flows into a transformer or a storage tank.
Good servicing and transformer oil treatment allows unlimited extention of its lifetime.