The service life of a transformer is directly dependent on paper insulation and the magnetic elements conditions. The magnetic elements have no limits on their service life, but paper insulation is exposed to stress and wear over time. Its condition has a great impact on term of a transformer service life. Aging of a paper insulation it is a long time process, but it can be significantly accelerated by heat, oxygen, water and the products of transformer oil oxidation, that accumulate in the paper over time.

Paper insulation in a transformer functions not only on its direct appointment, but also has a feature of a filter and accumulates the products of transformer oil aging. Changing the oil in the transformer doesn’t solve the problem. As cellulose insulation still contains all the contaminants, they start to dissolve in new oil as early as oil is changed. Contaminants cause oxidation of oil and precipitate the forming of new deposits in the paper insulation, so the cycle repeats over and over again. Moreover, changing the oil is not the best and  the most effective solution. Even if oil is taken out of the transformer, being regenerated and returned into the transformer tank, it doesn’t mean that the result will be different from the simple oil change. The only advantage of this process is its lower cost. Flushing the paper with clean oil, during transformer overhaul, can remove much of the contaminants, however, there are no guarantees that the oxidation products deposits will be removed. Using of special detergents may also help, but the process is longer and more expensive. Anyway, repairing the transformer and washing the paper insulation can cost as much as one third of an average new transformer price.