Transformer oil composition is very complex, which is mainly composed of alkanes and cycloalkanes, aromatics, transformer oil and its relative dielectric constant between 2.2-2.4, transformer oil pure dielectric strength is very high, can reach more than 4000kV/cm, but works with the purification of transformer oil, only at 50 ~ 60kV / 2.5mm, this is mainly because there are impurities, moisture, air bubbles mixed inevitably in the manufacturing and operation process, and in the operation of electric field and thermal decomposition of gas and oil, polymer.
In high electric field, the decomposition of the gas, and the oil in the water and fiber and other impurities in the electric field, the electric field along the direction, arranged in a "bridge", a leakage channel, in serious cases, lead to "bridge" breakdown, to reduce oil pressure strength. Therefore, the structure of transformer internal insulation should take into account the above factors, take necessary measures to prevent the formation of "small bridge"".
Thermal aging is present in all transformer oils, where the original residual oxygen is produced and the oxygen is produced when the fiber is broken down. When the operation temperature is high, the oxidation process of transformer oil is faster, which makes the viscosity increase, the color becomes deeper, the mud increases, the TG delta value increases, and the breakdown voltage decreases. In addition, there is also the problem of electrical aging. As the pressure time increases, the breakdown voltage of the oil gap decreases.
In oil immersed power transformers, partial discharges occur at high field strengths, which promote the condensation of oil molecules into more highly polymeric waxes, while escaping low molecular gases. The wax material accumulates in the winding insulation near the high field strength, blocking the oil passage, affecting the heat dissipation, increasing the gas generated, and discharging more easily.