Electrical power cables is use for the transmission and distribution of electrical energy is call electrical power cable. Power cable consists two or more electrical conductors join with an over sheath. It is use for the transmission of extra high voltages in a place where overhead lines are impracticable to use like, the sea, airfield crossing, etc. But underground cable is more costly as compared to aerial cable for the same voltage which is one of the main draws back of electrical power cable.
Construction of Electrical Power Cable
These are various parts of a cable according to construction. The power cable mainly consists of
- LAY for Multi core cables only
- Beading/Armoring (if required)
- Outer Sheath
Coppers and aluminum wires is use as a conductor material in cables because of their high electrical conductivity. Solid or number of bare wires made of either copper or aluminum is use to make a power cable.
For a conductor having more than three wires, the wire is arrange around a center wire such that there are six in the first layer, twelve in the second, eighteen in the third, and so on. The number of wires in the conductors are 7, 19, 37, 61, 91, etc., The size of the conductor is represent by 7/A, 19/B, 37/C, etc., in which first figures represent the number of strands and the second figure A, B, C, etc., represents the diameters in cm or mm of the individual wire of the conductors.
The most commonly use dielectric in power cables is impregnate paper, butyl rubber, polyvinyl chloride cable, polyethylene, cross-linked polyethylene. Paper insulated cables are mostly preferred because their current carrying capacity is high, generally reliable and having a long life. The dielectric compound used for the cable should have following properties.
- The insulator must have high insulation resistance.
- It should have high dielectric strength so that it does not allow the leakage current to pass through it.
- The material must have good mechanical strength.
- The dielectric material should be capable of operating at high temperature.
- It should have low thermal resistance.
- It should have a low power factor.
The cables used for submarine and damp soil should use synthetic dielectrics like polyvinyl chloride, polyethylene, etc. These materials are comparatively lighter and have nonmigratory dielectric. Also, such type of dielectric material has good dielectric strength, low power loss, and low thermal resistance.
It is use for protecting the cable from moisture’s which would affect the insulation. Cable sheath is made of lead alloy, and these strengths withstand the internal pressures of the pressurize cables. The material used for inner sheath should be nonmagnetic material.
The aluminum sheath is also use in a power cable because it is cheaper, smaller in weight and high mechanical strength than the lead sheath. In oil-filled cables and telephone, cables corrugate seamless aluminum sheath is use because it has better-bending properties, reduce thickness, and lesser weight.
Lead sheath cables when directly laid down on the ground are damage by corrosion and electrolyte. For protecting the cables against corrosion layers of fibrous material like paper, hessian, etc., or polyvinyl chloride is use. Layers of fibrous material spread with the waterproof compound to the outside of the electrical cable are called serving.
Armoring is the process in which layers of galvanize steel wires or two layers of metal tape are apply over sheath for protecting it from mechanical damage.The steel wires are normally use for armoring because it has high longitudinal strength. Armoring is also use for earthing the cable. When the fault occurs in the cable (due to insulation failure) the fault current flows through the Armour and get earth.
It gives the mechanical strength to the cables. It protects the cable from overall damage like moisture, corrosion, dirt, dust, etc. The thermo setting or thermoplastic material is used for making over the sheath.