PCBs were among the best and most widely used synthetic insulating liquids for electrical machines, such as power ca­pacitors and transformers, due to their superb electrical char­acteristics and nonflammability, until a total ban on their use and production was imposed, first in Japan in 1972, then in the USA in 1976, and then in Europe in 1985.

In the 1960s alkylbenzenes were initially developed for high-voltage cables in view of their superior gassing proper­ties under high voltage stress, and especially for use with syn­thetic paper.

At the time PCBs were banned, other kinds of synthetic aromatic hydrocarbons such as alkylnaphthalenes and alkyl- diphenylethanes had been developed as candidates for im­provements on mineral oils, but because of their higher cost, they had not been put into practical use. PCBs were then re­placed mainly by these new aromatic hydrocarbons.

Aromatic Hydrocarbons

Alkylbenzenes consist of a benzene ring and an alkyl group of the straight-chain or branched-chain type. Alkylbenzenes are

Table 6. Characteristics of Mineral Transformer Oils


Paraffinic Oil 1

Paraffinic Oil 2b



Flash point (°C)




Kinematic viscosity



7.8 X 10—6

9.6 X 10—6

7.9 X 10—6


2.2 X 10—6

2.5 X 10—6

2.1 X 10—6

Pour point (°C)

— 25








tan S (%)





p (П • m)





a tan S = dissipation factor; p = volume resistivity. b Containing azkylbenzene and pour-point depressant.

mainly used for high-voltage cables, including cables with synthetic paper, as they have excellent compatibility with plastics. They can be blended with mineral oil to improve its gassing properties and thermal stability.

In IEC 60867, alkylbenzenes are divided into three classes according to their viscosity and flash point.

Alkyldiphenylethane (phenylxylylethane), alkylnaphtha – lene (diisopropylnaphthalene), and methylpolyarylmethanes (blends of benzyl – and dibenzyltoluene and blends of benzylto – luene and diphenylethane) are mainly used for high-voltage power capacitors and also used for instrument transformers. Because all of these liquids consist of two benzene rings with much shorter-chain alkyl groups than in alkylbenzene, their aromatic contents are higher than those of alkylbenzene, and their resistance to partial discharge is very high. They have excellent dielectric properties and also good compatibility with plastic film, especially with the polypropylene film cur­rently used for capacitors; but all of them have lower per­mittivity (2.2 to 2.5) and flash point (130°C to 150°C) than PCBs. In some cases, they are stabilized by epoxiside or anti­oxidant. Their properties are also specified in IEC 60867.

Silicone Liquids

The silicone liquids currently used for electrical machines are polydimethylsiloxanes and have a variety of viscosities and flash points. Properties of a silicone liquid with a kinematic viscosity of 40 mm2/s at 40°C are specified in IEC 60836. They are used mainly for special transformers, due to their good thermal stability at higher temperature and better flow at lower temperature than mineral oil and because they are not very flammable. They are also sometimes used for capacitors and cables.

Organic Esters

Dioctylphthalate (DOP) and diisononylphthalate (DINP) have been used as substitutes for PCBs, especially for capacitors, because they have higher permittivity (4.5 to 5.5) and flash point (200°C to 240°C) than aromatic hydrocarbons. Di-2- ethylhexyl orthophthalate (DOP) is specified in IEC 61099 as a capacitor ester (type C1). As not easily flammable liquids, phosphoric acid esters such as tricresyl phosphate (TCP) and trixylenyl phosphate (TXP) are used as blends with aromatic hydrocarbons. Generally speaking, these esters have high permittivity and high inherent resistance to electrical stress, but as manufactured they contain much water and impurities and their dielectric dissipation factor is very high, so they must be carefully dehydrated and purified before impregna­tion and often need an antioxidant or scavenger.

Recently, organic tetraester liquids have been introduced in transformers because they are less flammable. Their fire point is higher than 300°C, but their viscosity is low compared with that of currently used mineral oils. The same precau­tions should be followed as mentioned above, and additives are effective as in the case of other organic esters.

Tetrahydric alcohol and a mixture of monocarboxylic acid with suitable stabilizing additives are also specified in IEC 61099 (type T1).


Polybutenes can have a large range of viscosity (1 mm2/s to 105 mm2/s at 40°C), depending on polymerization. These liq­uids are now used mainly in hollow cables (pipe-type cables) and to some extent in low-voltage capacitors. Polybutenes are specified in IEC 963 and classified into three classes, mainly depending on their viscosity. They are selected according to the specific application.

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