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A Brief Guide To Understanding Transformer Oil Deterioration

Just like it is for any other industrial equipment, be it a motor or generator – when a sign of decline is seen in its performance ability – there may be a faulty part. Usually, that’s when it’s sent to your trusted engineer for electric motor or generator overhauling for a thorough check and diagnosis of the exact issue.

Meanwhile, in a transformer’s case – it could be due to various problems like overheating, excessive loads, or even poor maintenance of accessories. Or it could be a result of oil deterioration, one of the leading causes present in this equipment.

But what’s responsible for transformer oil deterioration? And what are the consequences involved when it occurs?

Below, we give an explanation of what you need to know about this issue in your transformer.

Transformer oil deterioration

Before we go into the causes of oil deterioration, one must first understand what oil degradation is.

In simple terms, it’s defined as the detrimental chemical and physical processes in the insulating liquid over its lifespan that has a significantly negative impact on its life expectancy and performance. It occurs when oxidation and ageing are present, and the oil produces unwelcomed products like sludges, acids, moisture, and so on.

This may be because certain applications will have its oil content be in contact with air, making it prone to oxidation and even accelerates due to the appearance of catalysts.

As such, the oil turns darker in colour, and the acid within it starts to increase, henceforth, causing a higher buildup of sludge and increasing other kinds of electrical properties like dissipation factor tan d, damaging the transformer and reducing its lifespan.

Causes: Moisture and oxidation

While deterioration can be caused by numerous factors like overheating, contamination, oxidation, and electrical stress – there are two specific main issues that result in degradation – moisture and oxidation.

Moisture contamination

Otherwise known as water, they appear as tiny droplets (emulsion) in the transformer oil, or free water at the bottom (demulsified water).

Both the demulsified and emulsified form of liquid have highly negative effects on the insulating properties of the oil as the dielectric strength of the water will decrease. In certain situations, some of the dissolved water may also be absorbed by the paper insulation of the transformer over time, which adversely affects the insulation’s properties.

Moreover, the liquid content within the paper insulation can be particularly hard to remove – drying it out with vacuum and heat may be necessary.


Though a less common type of degradation, it may still occur. While compared to moisture, it is known to occur much slower, but the detrimental effects afterwards are more severe.

Oxidation causes an increased formation of sludge and the appearance acids within the oil. The rate of oxidation is highly dependent on temperature, when transformers are used during high temperature climates – they become even more prone to such oxidation. High temperatures and acid content will speed up the degradation of oil’s insulating properties and will cause the transformer to break down.

So, for oxidation to be kept at a minimum, oxygen must be removed from the transformer. The reason being that oxygen is able to come into contact with the transformer’s inner parts as oxygen occurring in moisture and atmospheric oxygen.

Hence, seals and gaskets have to be maintained closely and regularly, while the liquid content has to at low levels.

How transformer oil testing comes into the picture

Known as a mineral-based oil commonly seen in the usage of transformers due to its dielectric strength and chemical properties – transformer oil functions as a cooling agent and insulator.

It has to be tested regularly as the oil will eventually deteriorate over time, and it’ll potentially lead to faults and costly maintenance.

So, whether it be through a preventive maintenance program or testing – or a combination of both – they’ll equally help in avoiding unwanted downtime and look into your transformer’s condition to determine any need for repair.

Now that you have a better idea of what transformer oil deterioration, it’s important to have your engineer conduct regular tests to verify its condition.

Meanwhile, ensure other such equipment like your generator is fully operational at all times too. Otherwise, when faulty – request for generator rewinding for any slight repairs before deciding to replace it with new equipment. All in all, always be one step ahead with regular maintenance or the proper programs to ensure your equipment performance is at its best.

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