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.

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