Synthetic Transformer Oils

Ordinarily flash points of mineral transformer oils are around 150°C. Therefore, mineral oils are not so desirable for trans­formers in trains and indoor substations. For those uses it is desirable to use nonflammable or less-flammable transformer oils. PCBs are nonflammable and are the most desirable oils for such applications. However, PCBs are no longer environ­mentally acceptable. Since they were banned, no transformer oils have been found that have the desired nonflammability.

Silicone (polydimethylsiloxiane) liquids have been put into use. These liquids (described in IEC 60836) have high fire points and good oxidation resistance, and are classified as less-flammable liquids in the National Electrical Code in the USA. They are often used for transformers of trains, and in some countries they have been used for distribution trans­formers.

Some polyolester liquids (described in IEC 61099) are used for transformer oils on account of their good thermal stability and low hydrolysis in the presence of water. Midel 7131 (The Micanite and Insulators Co.) and Enviro Temp 100 (RTE Co.) are examples. Mixtures of flon 112 and tetrachloroethylene such as Formel. NF (ISC Chemicals Ltd.) have been developed for transformer use. This liquid has environmental problems because of the flon 112. However, tetrachloroethylene is non­flammable, and it and its mixtures with mineral oils have been classified as nonflammable by Factory Mutual.

As previously mentioned, high-molecular-weight hydrocar­bons with fire point higher than 300°C are classified as less- flammable oils in the National Electrical Code and are used for transformer oils. In Table 7 properties of some trans­former oils are shown.

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