Oil-Filled Cable

a high-voltage power cable in which the paper insulation is impregnated with mineral oil under pressure. An increase in the electric strength of insulation is achieved in oil-filled cables through the elimination of gas inclusions (voids) within the insulation, which are potential sites of breakdown, by filling them with oil. During operation of the cable the oil pressure is maintained by make-up equipment. Oil-filled cables are used to lead power lines from large power plants or underground hydroelectric power plants to distribution equipment, where power transmission lines cross water obstacles, in densely builtup areas, and where power lines extend far into cities with high power consumption.

Two types of oil-filled cable are manufactured in the USSR. The first type is a single cable with a centrally located oil channel, with the oil under low or medium pressure (0.1-0.3 meganewtons per sq m [MN/m²]). The second type is a multicore high-pressure cable (1.4-1.5 MN/m²). The first type has a conductor with a cross section of 150-800 sq mm made of tinned, shaped copper wires arranged in concentric layers. The channel diameter is usually 12 mm for cables of any cross section; it is formed by twisting the wires of one layer. The insulation of an oil-filled cable consists of oil-impregnated high-voltage cable paper (sometimes calendered); the electric strength of the oil is not less than 180 kilovolts per cm (kV/cm). The insulation is separated from the strand and the metallic sheath by a screening layer of semiconducting paper. Sometimes the outer screen is supplemented by copper or aluminum foil. The lead sheathing is usually reinforced by a hard-rolled copper ribbon. The use of aluminum sheathing substantially decreases the cost of oil-filled cable and also reduces its weight. However, the aluminum sheathing must be corrugated to make it flexible, and it also requires increased protection from corrosion. Single oil-filled cable is most frequently used for voltages from 110 to 220 kV.

In multicore high-pressure oil-filled cables the insulated, circular multiwire conductors have no inner channel; they are arranged in an oil-filled steel pipe with a diameter of 220 to 270 mm. The paper insulation and the cross sections of the strands are the same as those of single cable, but the electric strength of insulation is considerably higher than in low-pressure oil-filled cables. The steel pipe is protected on the outside by corrosion-resistant coatings. The cable is assembled directly at the cable run. The pipeline is welded together from sections; the insulated conductors are delivered from the factory in a lead sheath, which is stripped when the conductors are pulled into the pipe. After completion of the laying operations, oil is pumped through the pipeline a number of times until the desired electrical characteristics are produced. High-pressure oil-filled cables are used for voltages from 220 to 750 kV. Forced cooling of the cable is desirable for voltages above 500 kV; such cooling is achieved by circulating cooled and purified oil through the pipeline.

In the USA and Japan, and in several European countries, oil-filled three-core cables are manufactured for voltages from 60 to 110 kV, with conductor cross sections from 80 to 325 sq mm. The conductors are arranged in a straight line (flat cable) or around the circumference. Oil-filled cables are noted for their high reliability and ability to withstand prolonged overloads, as well as for the stability of the electric strength of the insulation.