Transformer oil is an insulating or cooling oil. It comes from crude petroleum. It is a mix of different hydrocarbons that are mainly aliphatic substances (open chain compounds) using the formula of CnH2n+2 and CnH2n. 

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Transformer oils - Nynas

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Here are some different types of oils that are used

Synthetic and mineral oil are the main transformer oils.

Mineral Oils

These are petroleum products, such as Naphthenic base transformer oil and Paraffinic based transformer oil. Naphthenic-based transformer oil is popular for its heat distribution which is among the most common issues associated that transformers face. They also have a nice fluidity feature when temperatures are low and are wax-free. 

These kinds of oils are best suited for use in low temperatures. While it does oxidize more easily and produces more sludge, the product that is formed through the process (i.e"sludge") is liquid. Thus, it doesn't hinder your transformer's cooling process.

Paraffinic-based transformer oil is made by paraffinic crude oil by solvent separation techniques. It is renowned for its excellent thermal and oxidation endurance as well as its high-temperature viscosity. Due to the high index of viscosity owing to it being a wax even though the rate of oxidation is lower than naphthenic oil, the precipitant, or the sludge develops as a result of the process of oxidation. 

Synthetic oils

Synthetic oils that are typically silicon-based were very popular in the mid-70s. It is typically utilized in areas with a high risk of fire due to its fire-retardant qualities. There are also a few issues of poor heat dissipation and large capacity for absorbing moisture. It's also more expensive in comparison to mineral oils.