Underground cables are used to transmit or distribute electric power. Mainly overhead system or underground system are used to transmit or distribute electric power. An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover.
Advantages of Underground Cables
undergrounds cables has many advantages compared to overhead system such as:
- Less liable to damage through storms or lightning
- Low maintenance cost
- Less chance of faults
- Lesser voltage drop and
- Better general appearances
Drawbacks of Underground Cable
Underground cables has following drawbacks:
- Greater installation cost
- Insulation problems at high voltages
Underground cables are employed where it is impracticable to use overhead lines. Underground cables are mainly used in thickly populated areas where overhead systems are prohibited due to safety purposes or around plants and substations or where maintenance conditions do not permit the use of overhead construction.
Construction of Underground cables
- Cores or conductors: A cable may have one or more than one core(conductor) depending upon the type of service for which it is intended. The conductors are made of tinned copper or aluminum and are usually stranded to provide flexibility to the cable.
- Insulation: Each core or conductor is provided with a suitable thickness of insulation the thickness of layer depending upon the voltage to be withstood by the cable. The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral compound.
- Metallic sheath: Metallic sheaths are generally used to protect the cable from moisture, gases, or other damaging liquids(acids or alkalis) in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation.
- Bedding: The layer of bedding is used over the metallic sheath which consists of a fibrous material like jute or hessian tape. The use of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.
- Armouring: Armouring is provided over the bedding which consists of one or two layers of galvanized steel wire or steel tape. The purpose of armoring is to protect the cable from mechanical injury while laying it and during handling.
- Serving: A layer of fibrous material(like jute) similar to bedding provided over the armouring is called serving. Serving is used to protect armouring from atmospheric conditions.
Classification of Underground cables
Mainly the cables for underground service is classified in two ways according to:
- the type of insulating material used in their manufacture
- the voltage for which they are manufactured
Cables are classified as:
- Low tension(L.T) cables -up to 1000V
- High tension(H.T) cables-up to 11,000V
- Super-tension(S.T) cables -from 22 kV to 33 kV
- Extra high-tension(E.H.T) cables from 33kV to 66 kV
- Extra super voltage cables -beyond 132 kV
Features of cables
- Insulation resistance of cable
- Capacitance of a cable
- Dielectric stress in a cable
- Most economical conductor size in a cable
- Grading of cables (Capacitance grading and intersheath grading)
- Cable faults (open-circuit fault ,short-circuit fault and Earth fault)
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