What is underground cable system? types, advantages & disadvantages
Underground Cables
An underground cable essentially consists of one or more conductors covered with suitable insula tion and surrounded by a protecting cover .
Although several types of cables are available , the type of cable to be used will depend upon the working voltage and service requirements . In general , a cable must fulfill the following necessary requirements :
( i ) The conductor used in cables should be tinned stranded copper or aluminium of high con ductivity . Stranding is done so that conductor may become flexible and carry more current .
( ii ) The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within permissible limits .
( iii ) The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed .
( iv ) The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it .
( v ) The materials used in the
manufacture of cables should be such that there is complete chemical and
physical stability throughout .
Construction of Cables
shows the general construction of a 3 - conductor cable . The various parts are :
( i ) 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 . For instance , the 3 - conductor cable shown in Fig . 11.1 is used for 3 - phase service . The conductors are made of tinned copper or alu minium and are usually stranded in order to provide flexibility to the cable .
( ii ) Insulatian . 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 .
( iv ) Bedding . Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape . The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring .
( v ) Armouring . Over the bedding , armouring is provided which consists of one or two layers of galvanised steel wire or steel tape . Its purpose is to protect the cable from mechanical injury while laying it and during the course of handling . Armouring may not be done in the case of some cables .
( vi ) Serving . In order to protect armouring from atmospheric conditions , a layer of fibrous material ( like jute ) similar to bedding is provided over the armouring . This is known as serving . It may not be out of place to mention here that bedding , armouring and serving are only applied to the cables for the protection of conductor insulation and to protect the metallic sheath from mechanical injury .
Insulating Materials for Cables
The satisfactory operation of a cable depends to a great extent upon the charac teristics of insulation used . Therefore , the proper choice of insulating material for cables is of considerable importance . In general , the insulating materials used in cables should have the following properties :
( i ) High insulation resistance to avoid leakage current .
( ii ) High dielectric strength to avoid electrical breakdown of the cable .
( iii ) High mechanical strength to withstand the mechanical handling of cables .
( iv ) Non - hygroscopic i.e. , it should not absorb moisture from air or soil . The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable . In case the insulating material is hygroscopic it must be enclosed in a waterproof covering like lead sheath .
( v ) Non - inflammable .
( vi ) Low cost so as to make the underground system a viable proposition .
( vii ) Unaffected by acids and alkalies to avoid any chemical action .
Classification of
Cables
Cables for underground service may be classified in two ways according to ( i ) the type of insulating material used in their manufacture ( ii ) the voltage for which they are manufactured . However , the latter method of classification is generally preferred , according to which cables can be divided into the following groups :
( i ) Low - tension ( L.T. ) cables - upto 1000 V
( ii ) High - tension ( H.T. ) cables -
( iii ) Super - tension ( S.T. ) cables
( iv ) Extra high - tension ( E.H.T. ) cables
( v ) Extra super voltage cables -beyond 132 kV
A cable may have one or more than one core depending upon the type of service for which it is intended . It may be ( 1 ) single - core ( ii ) two - core ( iii ) three - core ( iv ) four - core etc. For a3 - phase service , either 3 - single - core cables or three - core cable can be used depending upon the operating voltage and load demand .
Laying of Underground Cables
The reliability of underground cable network depends to a considerable extent upon the proper lay and attachment of fittings i.e. , cable end boxes , joints , branch con nectors etc. There are three main methods of laying underground cables viz . , direct laying , draw - in system and the solid system .
1. Direct laying .
This method of laying underground cables is simple and cheap and is much favoured in modern practice . In this method , a trench of about 1-5 metres deep and 45 cm wide is dug . The trench is covered with a layer of fine sand ( of about 10 cm thickness ) and the cable is laid over this sand bed . The sand prevents the entry of moisture from the ground and thus protects the cable from decay . After the cable has been laid in the trench , is covered with another layer of sand of about 10 cm thickness . The trench is then covered with bricks and other materials in order to protect the cable from mechani cal injury . When more than one cable is to be laid in the same trench , a horizontal or vertical inter axial spacing of atleast 30 cm is provided in order to reduce the effect of mutual heating and also to ensure that a fault occurring on one cable does not damage the adjacent cable . Cables to be laid in this way must have serving of bituminised paper and hessian tape so as to provide protection against corrosion and electorlysis .
Advantages
( i ) It is a simple and less costly method .
( ii ) It gives the best conditions for dissipating the heat generated in the cables .
( iii ) It is a clean and safe method as the cable is invisible and free from external disturbances .
Disadvantages
( i ) The extension of load is possible only by a completely new excavation which may cost as much as the original work .
( ii ) The alterations in the cable netwok cannot be made easily .
( iii ) The maintenance cost is very high .
( iv ) Localisation of fault is difficult .
( v ) It cannot be used in congested areas where excavation is expensive and inconvenient .
2. Draw - in system .
In this method , conduit or duct of glazed stone or cast iron or concrete are
laid in the ground with manholes at suitable positions along the cable route .
The cables are then pulled into position from manholes . Fig . 11.11 shows
section through four - way underground duct line . Three of the ducts carry
transmis sion cables and the fourth duct carries relay protection con nection ,
pilot wires . Care must be taken that where the duct line changes direction ;
depths , dips and offsets be made with a very long radius or it will be
difficult to pull a large cable between the manholes . The distance between the
manholes should not be too long so as to simplify the pull ing in of the cables
. The cables to be laid in this way need not be armored but must be provided
with serving of hessian and jute in order to protect them when being pulled
into the ducts . Advantages
Advantages
(i)Repairs , alterations or additions to the cable network can be made without opening the ground . ( ii ) As the cables are not armoured , therefore , joints become simpler and maintenance cost is reduced considerably . ( iii ) There are very less chances of fault occurrence due to strong mechanical protection pro vided by the system .
Disadvantages
( i ) The initial cost is very high .
( ii ) The current carrying capacity of the cables is reduced due to the close grouping of cables and unfavourable conditions for dissipation of heat . This method of cable laying is suitable for congested areas where excavation is expensive and inconvenient , for once the conduits have been laid , repairs or alterations can be made without opening the ground . This method is generally used for short length cable routes such as in workshops road crossings where frequent digging is costlier or impossible .
3. Solid system .
In this method of laying , the cable is laid in open pipes or troughs dug out in earth along the cable route . The troughing is of cast iron , stoneware , asphalt or treated wood . After the cable is laid in position , the troughing is filled with a bituminous or asphaltic compound and covered over . Cables laid in this manner are usually plain lead covered because troughing affords good mechanical protection . Disadvantages ( i ) It is more expensive than direct laid system . ( ii ) It requires skilled labour and favourable weather conditions . iii ) Due to poor heat dissipation facilities , the current carrying capacity of the cable is reduced . In view of these disadvantages , this method of laying underground cables is rarely used now - a days .
Labels: power system
posted by Infotake @ November 17, 2021
0 Comments
0 Comments:
Post a Comment
Subscribe to Post Comments [Atom]
<< Home