Transformer oil is also referred to as insulating oil. This is a specialised product used to insulate and cool transformers, as well as other electrical equipment.
The colour of transformer oil is particularly important, as it can act as a diagnostic, which indicates the health of the transformer oil – helping maintenance teams keep equipment functioning properly and preventing failures.
In this article, we will run through the various colours that transformer oil can be, and their significance.
New oil
Fresh transformer oil – whether it’s mineral-based or synthetic – is usually clear or pale yellow. There are many varieties of transformer oil available, made by brands like Shell.
This clarity is a sign of pure low moisture content with a very low level of contaminants.
At this stage, oil is at its most efficient, safe to use and ideal for insulation and heat transfer.
Yellow / orange
Over time, transformer oil may develop a yellow or orange tint. This is generally a sign of oxidation beginning to occur. This shift in colour does not represent an immediate risk, but suggests that the oil is starting to degrade and should be monitored more closely.
At this stage, acids or sludge may also begin to form.
Brown / black
If a transformer oil has become brown or black, this is a clear warning sign.
Darkening of this kind usually indicates heavy contamination, and can be the result of high oxidisation, carbon particles from arcing and high moisture content.
A brown or black colour shows that the oil’s insulating properties are probably compromised. Transformer oil with this colour can lead to increased operating temperatures, sub-par performance and even complete failure.
Immediate testing, potentially followed by filtration and replacement are necessary.
Other colours
In unusual cases, a transformer oil may also turn other colours, such as green or milky white.
A green tint could suggest a chemical reaction with certain materials inside the transformer. Milky white or cloudy oil usually indicates water contamination; a problem that reduces dielectric strength and increases the risk of short circuits. These are issues that require urgent attention.
Colour as an indicator
Colour alone is not a definitive measure of transformer oil condition, but as we’ve outlined in this article, it is a highly visible early warning.
Proper transformer maintenance will involve regular inspections and analysis of oil, which include a close observation of colour.
