Monthly Archives: August 2014


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.