Good transformer oil should be a clean, transparent liquid with no deposits, suspended solids of mechanical impurities and cotton-like materials. If it is contaminated and oxidized, and produces resin and precipitate, the oil quality of the transformer will deteriorate and the color will gradually turn light red until it turns into a dark brown liquid.
When the transformer is faulty, the color of the oil will also change. Under normal circumstances, when the transformer oil is light brown, it is not suitable for reuse. In addition, the transformer oil can behave as a turbid milky, oily black, and dark. The transformer oil is cloudy and milky, indicating that the oil contains moisture. The oil color is dark, indicating that the transformer oil is aging. The oil color is black and even has a burnt smell, indicating that there is a malfunction inside the transformer.
GTD-61A oil tan delta tester/oil dielectric loss tester/oil dissipation factor and resistivity tester,let you know the performance of the insulation oil.