Transformer oil – manufactured by brands such as Nynas and Millers – is crucial for electrical power systems, providing insulation and cooling for transformer components.
When this oil changes from its original clear, pale-yellow colour to brown or dark amber, it signals potential problems. Understanding the cause of oil discolouration helps maintenance personnel take immediate action; identifying issues before they lead to transformer failure.
In this article, we will ask what causes transformer oil to go brown.
Oxidation: The primary culprit
Oxidation is the most common cause of transformer oil turning brown.
When oil comes into contact with oxygen – especially at elevated temperatures – it undergoes a chemical reaction that produces acidic compounds and sludge. It is these resulting products from oxidisation that create the brown discolouration, while also degrading the oil’s dielectric (being able to conduct electricity) properties.
When there is copper and iron present from transformer windings and core materials, this speeds the process up, and these chemical elements act as catalysts for oxidisation reactions.
Fresh transformer oil contains natural antioxidants that initially resist this degradation, but these compounds become depleted over time.
High temperature effects
Elevated operating temperatures significantly accelerate the degradation process of transformer oil.
Premature browning may occur in transformers that consistently operate above recommended temperature ranges. High temperatures increase the rate of oxidisation reactions, and this results in thermal stress and chemical degradation compounding each other.
Transformer temperatures can be pushed up – beyond safe operating limits – by overloading, poor ventilation or malfunctioning cooling systems.
Arcing and electrical breakdown
Internal arcing within transformers produces localised temperatures that can exceed 3000°C. This causes significant oil degradation.
These electrical faults create carbon particles and byproducts that contaminate the oil, contributing to brown discolouration.
Cellulose degradation
Transformer windings and internal structures contain cellulose-based insulation materials. These gradually degrade over time. As cellulose breaks down, it releases compounds that dissolve into the oil, contributing to brown discolouration.
External contaminants
Various contaminants can enter transformer oil systems and discolouration, as well as performance degradation.
Water, dust, metal particles from internal wear and residues from maintenance activities can all contribute to oil contamination.
Regular oil testing and filtration help identify and remove contaminants before they cause significant damage. This preserves both oil quality and transformer reliability.
