Vegetable oils (castor oil, rapeseed oil, etc.) are basically tri­glyceryl esters of fatty acids, and the fatty acids can be satu­rated or unsaturated. They were once used for cables and ca­pacitors, and are now mostly used for the impregnation of dc capacitors and especially energy storage capacitors, as they have high permittivity. They have not been used for ac power capacitors, as they have poor dielectric dissipation factors. Re­cently, however, they have been tried for use with metallized polypropylene films, with which they have good compatibility, and their dissipation factor and gas-absorbing ability have been improved by blending them with aromatic hydrocarbon liquids.


Transformers were developed and began to be manufactured in the mid 1880s in Hungary, the USA, the United Kingdom, and France. In the years 1886 to 1891, manufacturers began to use oils for insulation transformers. Such oils (transformer oils) are specified in IEC 60296 and 60836 and in ASTM D3487 and D4652.

Transformer oils must have the following properties:

1. High dielectric strength and low dielectric losses

2. Good cooling power (mainly dependent on viscosity)

3. High chemical stability and high resistance to oxidation

4. Good compatibility with insulating materials

5. Low corrosive sulfur content

6. Low viscosity and good fluidity over a wide temperature range (low pour point)

7. Sufficient source of supply

8. High flash point

9. Nontoxicity

Of these, properties 1 and 2 are most important from the viewpoint of transformer performances.

Mineral Transformer Oils

Mineral oils have been used as transformer oils since the be­ginning of their manufacture. When properly refined, mineral oils have various excellent properties mentioned above. At present, mineral oils are used over wide range of transformer capacity, from distribution transformers to ultrahigh-voltage transformers.

Mineral oils are manufactured by refining crude oils. De­pending on the composition of the crude oils, there are two kinds of mineral oils: naphthenic and paraffinic. The pour points of paraffinic oils are generally higher than those of naphthenic oils.

Sometimes mineral oils are mixed with each other or with other oils. The mixtures may be between oils of the same type, between naphthenic and paraffinic oils, or with nonmineral oils. Some specifications can be found in IEC 60296. In some countries, mixtures of mineral oils and alkylbenzenes are used as transformer oils. Such oils have high resistance to oxidation, low corrosion, and low pour point. In the case of paraffinic oils, because of their relatively high pour points, pour-point depressants are added.

Because oils are oxidized under air, small amounts of anti­oxidants are added to some mineral oils, especially in Europe and North America. Such mineral oils are classified in IEC 60296 and ASTM D3487. However, in some countries mineral oils with antioxidants are not used.

In 1970s some flashover faults were found in ultrahigh – voltage transformers due to flow-induced electrification (streaming electrification). Factors that affect this phenome­non are transformer design (especially the flow rate of the oil), oil temperature, and properties of the insulating oils such as the volume resistivity and electrostatic charging tendency. Flow rates of oil have been controlled in some transformers to suppress this phenomenon. It is said that 1,2,3-benzotriazol (BTA), which has been known as a deactivator agent for met­als, suppresses this phenomenon. In some countries a small amount of BTA has been added to mineral oils for high-volt­age and high-power transformers for that purpose.

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