Transformer oil is mostly mineral insulating oil, which is a product of petroleum fractionation. Its main components are alkanes, naphthenic saturated hydrocarbons, aromatic unsaturated hydrocarbons and other compounds.

良好的变压器油应该是清洁而透明的液体，不得有沉淀物、机械杂质悬浮物及棉絮状物质。如果其受污染和氧化，并产生树脂和沉淀物，变压器油油质就会劣化，颜色会逐渐变为浅红色，直至变为深褐色的液体。

Good transformer oil should be clean and transparent liquid, free of sediment, mechanical impurities, suspended solids and cotton flocculent substances. If it is polluted and oxidized, and resin and sediment are produced, the quality of transformer oil will deteriorate, and the color will gradually change to light red until it turns to dark brown liquid.
                 

当变压器有故障时，也会使油的颜色发生改变一般情况下，变压器油呈浅褐色时就不宜再用了。另外，变压器油可表现为浑浊乳状、油色发黑、发暗。变压器油浑浊乳状，表明油中含有水分。油色发暗，表明变压器油绝缘老化。油色发黑，甚至有焦臭味，表明变压器内部有故障。

When there is a fault in the transformer, the color of the oil will also change. Generally, when the transformer oil is light brown, it is not suitable to use it again. In addition, transformer oil can be shown as turbid emulsion, black and dark. The turbid emulsion of transformer oil indicates that there is moisture in the oil. The darkening of oil indicates that the oil insulation of transformer is aging. The oil turns black and even has a scorching smell, indicating that there is a fault inside the transformer.