Underground Cable: Defination, construction & Desirable properties

(Define the underground cable.Explain the construction of underground cable and it's properties.)

Underground cables:

The transmission and distribution of an electrical power can be done with the help of overhead transmission line or by underground cables. In dense populated area like cities, the use of overhead transmission line is not practicable and in such a case we can use underground power cables. The underground cables of course come with other amenities, the advantage include smaller voltage drops and lesser probability of fault occurrence. On the other hand their production and installation cost is higher and used where overhead lines aren't

viable due to practical limitations or risk involved. 

A typical underground cable consists of a conductor's covered by a number of insulating and protecting layers necessary for its satisfactory operation. 

Construction of Underground Cables:

Conductor:

Usually one, three or more (depending upon the application) conductors are used. These conductors are stranded to reduce skin effect, proximity effect and to keep it flexible. 

Conductors are made from electrolytic grade pure copper or aluminum materials. Conductors are also called cores. A cable with

three conductor is called three core cable.

Insulation:

It is provided to withstand the electrostatic stress. Various types of insulators VAR (Vulcanized India Rubber), Impregnated paper, PVC and XLEP (cross linked polyethylene) are utilized depending upon applied voltage. 

Metallic sheath:

In order to protect cable from moisture, gases or other acids and alkalies present in soil and surrounding, a metallic sheath made of lead or aluminum is used over insulation screen. It also provides a path for fault and leakage currents as the sheath is earthed at one cable end.

Bedding:

It is a low grade insulator like hessian (fibrous material) which protects

the metallic sheath from corrosion and from mechanical injury due to

armouring.

Armouring:

It provides mechanical protection from various stresses the cable may get exposed to during its installation and operational life. It's usually a steel tape wound around the bedding layer.It also holds the fibrous bedding attached with the metallic sheath.

Serving

Another layer of low grade insulator like jute or hessian or thermoplastic compound like PVC is again provided to protect the steel(armouring) from atmospheric contaminants and agents.

Note: It should be noted that bedding, armouring and serving

are only applied to cables for the protection of the conductor insulation and to protect the metallic sheath from mechanical injury.

Requirements of cables:

Many types of underground cables are available depending on the nature of conductors, number of conductors, types of insulation used etc. The basic necessary requirements of the cables are:

 The size of conductor used must be such that it should carry the specified load without overheating and keeping the voltage drop well within permissible limits. 

 At the voltage level for which cables are designed the insulation thickness must be proper so as to provide high degree of safety and reliability.

 The cable must be surrounded by a number of layers of an additional insulation so as to give proper mechanicalstrength and protection.

 The materials used in the manufacturing of cables must be such that there is a complete chemical and physical stability. Properties of Insulating Materials insulating materials used for cables should have good insulation

properties tha t are physically positioned directly over the conductor or the conductor shield. The higher the cable voltage the thicker is the insulation required and greater is the cable size.

Some most important properties of insulating materials used for power cables are given as:

 High resistivity to avoid leakage current

 High dielectric strength to avoid electrical breakdown of cables

 Low thermal coefficient

 Low water absorption/ less hygroscopic

 Low permittivity

 Non inflammable

 Chemicalstability

 High mechanical strength