The capacitor is one of the oldest electrical components. An oil-immersed capacitor was developed in the 1850s, but the fundamental technologies of modern oil-impregnated and oil – filled high-voltage power capacitors originated with those of high-voltage power cables.

After PCBs were developed around 1930, PCBs (mainly trichlorobiphenyl for high-voltage power capacitors) were used for can-type capacitors and mineral oils were used for large tank-type capacitors, until PCBs were recognized as en­vironmentally hazardous. PCBs were also used for a short pe­riod as the impregnant for plastic film (polypropylene) dielec­trics of both paper-film and all-film types. Just after the ban on PCBs, those impregnants were replaced by aromatic hy­drocarbons. As the aromatic contents of these hydrocarbons are very high, they are very suitable for the impregnation of high-voltage capacitors with sharp-edged foil electrodes.

Because aromatic hydrocarbons are more flammable than PCBs, silicone or blended oils of aromatic hydrocarbons and phosphoric acid esters have been used for high-voltage capaci­tors as less-flammable liquids for limited use; but recently dry capacitors have been developed for use where fire-resistant materials are strictly required.

To minimize the dielectric thickness, self-healing technol­ogy originally developed for low-voltage capacitors has re­cently been applied also for high-voltage capacitors. In this case, as metallized paper or film is used, and therefore com­patibility between the impregnant and the solid material is very important, impregnants such as organic esters are used.

Liquids for high-voltage power capacitors are specified in IEC 60836 (silicone liquids), 60867 (aromatic hydrocarbons), and 61099 (organic esters).

Liquids for high-voltage capacitors must have the follow­ing properties:

1. High dielectric strength and high volume resistivity

2. Low dielectric losses and high dielectric constant

3. High stability under high voltage stresses and high par­tial discharge resistance

4. Good compatibility with film materials

5. High chemical stability and high resistance to oxidation

6. Low temperature coefficient of expansion

7. Nontoxicity and environmental safety

8. Sufficient source of supply

Of these, properties 1, 2, and 3 are most important from the viewpoint of high-voltage capacitor performance.


Liquids for switchgear (switchgear oils) must have arc sup­pression properties and high dielectric strength. Arc suppres­sion properties are basically due to the high thermal conduc­tivity of hydrogen gas produced by the decomposition of switchgear oils. Thus it is desirable that liquids easily pro­duce hydrogen gas and that the amount of free carbon pro­duced by their decomposition be small. Good insulation, re­quires not only high dielectric strength, but also rapid insulation recovery after interruption of electric arcs.

Besides these properties, it is desirable that switchgear oils have chemical stability to maintain good dielectric prop­erties, and that they be compatible with the solids used. Insu­lating oils that have the above-mentioned properties are min­eral oils. Switchgear oils are specified in IEC 60296 and ASTM D387. They are classified in the same classes as trans­former oils.

The kinematic viscosities of insulating oils in these classes are relatively low: for insulating oils at 40°C classified in IEC 60296 as Class I and Class IA, Class II and Class IIA, and Class III and Class IIIA they are <16.5 X 10—6 m2/s, <11.0 X 10—6 m2/s, and <3.5 X 10—6 m2/s, respectively. The kine­matic viscosities of insulating oils at 40°C classified in ASTM D3487sa Type I and Type II are <12.0 X 10—6 m2/s. Low ki­netic viscosity allows mechanical parts of switchgears to per­form freely, and oil flows owing to hydrogen gas evolved by decomposition of switchgear oils to be easily produced and fa­cilitate arc suppression.


Oil-immersed power cables were developed and put into use in the 1880s, and a historic milestone in recent engineering and industrial progress was established by the invention and development of the oil-impregnated or oil-filled (OF) power cable by Emanuelli in 1923. OF cables are impregnated with oils without voids or moisture and then hermetically sealed to avoid damage and harmful effects from the surroundings.

From the early stage of OF cables, naphthenic oils have been mainly used because of their low pour point and high
stability under high stress, but with the progressive improve­ment of process technology for refining crude oil, paraffinic crude oils and mixtures of naphthenic and paraffinic oils have also been used because of their wider availability.

Aromatic content in mineral oil is also important, and in some cases synthetic aromatic hydrocarbons are added. Pure synthetic aromatic hydrocarbons, mainly alkylbenzenes, are also used, especially for ultrahigh-voltage power cables, be­cause of their compatibility with synthetic papers, excellent stability under high stress, and sufficient source of supply.

Polybutenes are used for hollow power cables because of their wide range of viscosity.

Liquids for cables are specified in IEC 60465 (mineral oils), 60836 (silicone liquids), 60867 (aromatic hydrocarbons), and 60963 (polybutenes).

Cable oils must have the following properties.

1. High dielectric strength and high volume resistivity

2. Low dielectric losses and low dielectric constant

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

4. High chemical stability and high resistance to oxidation

5. Low temperature coefficient of expansion

6. Sufficient source of supply

7. Nontoxicity and environmental safety

Of these, properties 1, 2, and 3 are most important from the viewpoint of power cable performance.

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.


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.