Regeneration of transformer oil at the transformer’s location is an important preventive measure of transformer servicing.
Transformer life time is, in essence, the life time of its insulation system. The most widely used insulation is liquid insulation (transformer oils) and solid insulation (paper, i.e. cellulose insulation). The oil provides for at least 80% of the electric strength in a transformer. Almost 85% of transformer failures are caused by damaged insulation.
Transformer oil is a good insulator when insulation paper is will impregnated: the oil increases the breakdown voltage of the insulation which it saturates. Low viscosity of the oil allows it to permeate solid insulation and dissipate heat by transferring it to the cooling system. Therefore, liquid insulation is also a cooling liquid. Oxidation stability of the oil allows it to endure high temperatures and prevent significant damage to the insulation system.
Aging or degradation of transformer oil is usually related to oxidation. As oxygen and water appear in the oil, the oil oxidizes even of other conditions are perfect. Contaminants generated by solid insulations also affect the quality of transformer oil. Reactions which occur in the oil between unstable hydrocarbons, oxygen and other catalyst, such as moisture, with such accelerators as heat, lead to oil decomposition (oxidation).
Heat and moisture, along with oxidation, act as primary accelerators of this process and are the largest threat to solid insulation. If the cooling and insulation system is serviced right, insulation system’s life time can be extended from 40 to 60 years. Unfortunately, oil oxidation cannot be entirely eliminated, however, it can be controlled and slowed by oil treatment. One of the most important transformer maintenance procedures is oil analysis scheduled at least annually. Oil analysis is indicative of the overall insulation condition.
Moisture is a combination of free water, water solved in the products of oil degradation, solved and chemically bound water (it is a part of glucose molecules and is necessary for maintaining the mechanical strength of cellulose). It is impossible to completely dehydrate cellulose insulation.
Transformer oil solves more water at higher temperatures. If the mixture of oil and water is cooled, water will settle out of the oil. The oil will permeate solid insulation, or become bound to oil degradation products. Moisture will distribute itself between the oil and the paper. However, this distribution will be uneven: paper absorbs water from the oil and retains it, in the areas of highest voltage.
Contaminants are formed in the process of transformer operation. Oil decay products are acidic, and they attack cellulose and metals; the acids also create soaps, aldehyde and alcohol, which settle on the insulation, tank walls, breathing and cooling systems in the form of sludge. Sludge forms faster in a heavily loaded, hot transformer operated incorrectly. Sludge increases oil viscosity thus reducing its cooling ability, which has further negative effect on transformer life time.
Contamination also causes insulation to shrink, destroys varnish and cellulose material. It is a conductor for discharges and currents; being hygroscopic, it absorbs moisture and leads to insulation overheating. Sediment forms on the core, which increases transformer temperature.
Cellulose material is the weakest link in the insulation system. Since transformer life time is in essence the life time of its cellulose insulation, and since cellulose degradation is irreversible, contaminants must be removed immediately, until they damage the cellulose. A good maintenance program extends its life time significantly.
Normal servicing of power transformer should attain a practical life time of 50 – 75 years. However, the actual condition of insulation defines the difference of real time operation between 20 – 50 years plus the transformer life time. Experience shows that the most common cause for transformer failure is inadequate servicing and incorrect operation.
Transformer oil can be completely regenerated and made as good as new. Insulation oil can be used indefinitely, if it is processed regularly. The prospect of regenerating a batch of very poor quality oil should be balanced against the relatively high cost of acquiring new oil.
Removal of water and keeping the insulation dry is of utmost importance. Moisture accelerates aging. 1% of moisture in cellulose accelerates aging by one order of magnitude in comparison to 0.1% moisture content.
So, what are the main guidelines for preventive maintenance?