Reliable operation of transformers is dependent on the condition of internal insulation. Transformers are insulated by liquid (oil) and solid (cellulose) insulation. Dielectric oil accounts for about 80% of the system’s dielectric strength. It is a very good dielectric, and it permeates cellulose insulation further increasing the dielectric strength of winding insulation material. Viscosity characterizes the oil’s cooling ability.
The oil must be protected from oxidation, avoiding overheating and contact with contaminants, since most of transformer failures are due to insulation failures.
Cellulose materials are the weak link of transformer insulation. Contamination of internal insulation by things like water, dissolved gas and solid particles cause degradation of insulation materials. Cellulose insulation quickly accumulates contaminants.
One of the main factors of transformer oil and solid insulation degradation is water. It inevitably enters insulation systems. The result of its influence (along with other contaminants) is aging of oil and formation of various degradation products, sludge, acids etc.
When oil absorbs air and moisture, the aging process occurs even under light loads. Water in transformer oil consists of free water, water solved in oil degradation products and chemically bound water. It is impossible to extract all water from solid insulation.
Transformer oil can accept more water at elevated temperatures. When oil is again cooled, this water precipitates form the oil and enters solid insulation. Cellulose insulation absorbs water form the oil and retains it. Acids resulting from oil aging have a negative effect on cellulose and metals and form soapy metal, aldehyde, alcohol etc which form acidic sediment on solid insulation, internal surfaces of the transformer, breathing system, cooling system etc.
While water and heat are damaging to solid insulation, correct maintenance of cooling and insulation system can extend their lifetime to 60 years. Equipment formerly used for transformer oil drying only is quite obsolete. At present oil can be restored entirely. The parameters of regenerated (reclaimed) oil are extremely close to those of new oil. Service life of well maintained transformer oil is virtually unlimited.
The modern comprehensive transformer oil purification plants include degassing, drying and lightening, as well as filtration and regeneration. The resulting regenerated product is as good as new. Therefore, new oil purchase costs are reduced. The cost of purchased and regenerated oil is recovered.
The cost of regeneration of used transformer oil is always considered in relation to the high cost of purchasing new resource and possible downtime costs. It is quite obvious that owners of complete transformer oil regeneration plants are always in advantage.