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.

在高电场中，这些分解出来的气体，以及油中的水分和纤维等杂质，在电场作用下，顺着电场方向，排列成“小桥”，成为泄漏的通道，情况严重时，导致“小桥”击穿，使油的耐压强度降低。因此，变压器内部绝缘的结构，要考虑上述因素，采取必要措施，防止形成“小桥”。

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"".

热老化在所有变压器油中都存在，油箱中既有原来残留的氧，而纤维分解时也会产生氧。运行温度较高时，变压器油的氧化过程就进行得比较快，使得粘度增高、颜色变深、泊泥增多、tgδ值增大、击穿电压下降等。另外，还存在着电老化的问题，随着加压时间的延长，油间隙的击穿电压下降。油浸电力变压器中，高场强处产生局部放电，促使油分子进一步互相缩合成更高分子量的腊状物质，同时逸出低分子的气体。腊状物质积聚于高场强区附近的绕组绝缘上，堵塞油道、影响散热、产生的气体增多，放电更易发展。

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.

因此在运行中需经常对油进行检查、试验，并及时进行处理(滤油等)。现在不少大型变压器采用充氮保护或隔膜保护措施。隔膜保护是用一略小于储油柜的耐油橡胶胶囊填充于储油柜的油面上，胶囊与大气相通，因而隔绝了变压器油与大气的接触。这样就保证了油性能的稳定。

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 an oil resistant rubber capsule which is slightly smaller than the oil conservator, and the capsule is communicated with the atmosphere, thus isolating the contact between the transformer oil and the atmosphere. This ensures the stability of the oil performance.