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 due to impurities, moisture, air bubbles into 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 internal margin of the transformer structure should consider the above factors and 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 a problem of electrical aging. The breakdown voltage of the oil gap decreases with the increase of the compression time. 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 high field field, the winding around it is close to the edge, blocking the oil passage, affecting the heat dissipation, increasing the gas produced, and the discharge is more easy to develop.
Therefore, the oil must be inspected, tested and handled in a timely manner. Now many large transformers adopt nitrogen protection or diaphragm protection measures. The diaphragm protection is filled with the oil resistant rubber capsule which is slightly smaller than the oil conservator, and the capsule is communicated with the atmosphere, so that the contact between the transformer oil and the atmosphere is separated. This ensures the stability of the oil performance.