The conditions of transformer oil use in modern oil-filled electrical equipment are quite harsh, due to strong electric fields and high temperatures.
The electric field in power transformers is created in horizontal oil channels in the windings, in oil gaps of main insulation and in the spaces between energized parts, such as windings, high voltage bushings and the transformer oil tank.
The strength of the electric field in the oil channel near the high voltage winding in some transformers reaches 70 kV/cm. In case of transformer oil filled bushings, used in 35 kV class equipment and above, oil switches and OTC devices, the mineral transformer oil is under the influence of strong electric fields.
There is a tendency of voltage growth in modern transformers, which requires improvement of winding insulation. The weight and dimension limitations due to the need transport transformer and reduce material costs, causes the reduction of insulation gaps, narrowing oil channels and increasing the strength of the electric field in them.
Considering the above, the strength of electric field in electrical equipment is one of the most important factors of rapid transformer oil aging. Therefore, the oil used in such equipment must be highly stable to high temperatures and oxidation. Transformer oil is subjected to heat and electric field, aging it, reducing its performance parameters due to the changing chemical composition of the oil.
The above notwithstanding, the lifetime of transformer oil should not be less than 10 years. When the quality of the oil drops to the low limits, the old oil is replaced with new and regenerated. The economic viability of the regeneration process is especially apparent today due to the rising costs and diminishing supply of petroleum products.