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.

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