Transformer oil – manufactured by brands such as Petronas and Nynas – naturally has a clear, pale appearance when fresh. Over time, it can develop a yellow tint due to age and degradation.
This colour change is a warning of oil deterioration, which indicates the need for close monitoring and, in some cases, immediate maintenance. With a knowledge of the discolouration’s causes, maintenance teams can help to keep transformers working reliably and extend the life of their equipment.
This article will discuss the causes of yellowing, and how maintenance teams can preserve transformer reliability and extend equipment life.
Oxidisation: The foundation of discolouration
Oxidisation is the main reason for transformer oil turning yellow.
When oil molecules react with oxygen, especially with heat and metal catalysts present, they form polar compounds and acids that produce the yellow appearance.
This process starts slowly, but continues as antioxidant additives become depleted over time.
The oxidisation process produces not only colour changes but also increases oil acidity and reduces dielectric (the ability to conduct electricity) strength. Copper and iron from transformer components accelerate these reactions.
Heat acceleration
Elevated temperatures significantly speed up all oil degradation processes. These include the process that turns transformer oil yellow.
Transformers that operate at higher load levels or in hot environments experience faster colour changes. This is because thermal energy drives chemical reactions more rapidly.
Effective cooling systems and proper loading practices help to minimise heat-related degradation.
Moisture contamination
Water ingress (the unwanted entry of water) accelerates oil degradation and contributes to yellowing.
Moisture promotes oxidation reactions, encourages the growth of microorganisms that produce acidic byproducts and allows the breakdown of cellulose insulation materials.
Once it has entered, water becomes difficult to remove completely and will affect oil quality throughout the transformer’s working life.
External contaminants
There are various contaminants that are introduced during manufacturing, installation or maintenance processes, contributing to oil yellowing. These include metal particles, dust, cleaning solvents and other foreign materials.
Impact on transformer performance
Yellow oil indicates reduced dielectric strength and an increased risk of electrical breakdown. The colour change also signals that the oil’s ability to dissipate heat effectively and provide adequate insulation has been decreased.
Oil analysis, monitoring and changes
Regular oil testing, along with understanding the importance of oil changes, can help provide a quantitative assessment of degradation beyond visual inspection. This involves tests measuring acidity, dielectric strength, interfacial tension and dissolved gas content.
Replacing degraded oil before severe discolouration is essential to maintaining transformer reliability and extending equipment life.
