undergroundcables-140530061534-phpapp01.pptx

7/25/2019 undergroundcables-140530061534-phpapp01.pptx

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Underground Cables

By

Anu SinglaAssociateProfessorDepartment ofElectricalEngineeringChitkara


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An underground cable consists of one or more

conductors covered !ith some suitable insulatingmaterial and surrounded by a protecting cover"

#he cable is laid underground to transmit electricpo!er"

Underground cables


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Although several types of cables are available, the type of cableto be used !ill depend upon the !orking voltage and service

re(uirements" $n general, a cable must ful)l the follo!ingnecessary re(uirements %

*i+ #he conductor used in cables should be tinned *coating the!ire !ith tin provides it !ith more durability and strength+

stranded copper or aluminium of high conductivity" Strandingis done so that conductor may become e-ible and carrymore current"

*ii+ #he conductor si'e should be such that the cable carries thedesired load current !ithout overheating and causes voltage

drop !ithin permissible limits"*iii+ #he cable must have proper thickness of insulation in orderto give high degree of safety and reliability at the voltage for!hich it is designed"

*iv+ #he cable must be provided !ith suitable mechanicalprotection so that it may !ithstand the rough use in laying it"

*v+ #he materials used in the manufacture of cables should be

Underground cables


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Construction of Cables

*iii+ 0etallic sheath" $n order to protectthe cable from moisture, gases or otherdamaging li(uids *acids or alkalies+ inthe soil and atmosphere, a metallicsheath of lead or aluminium is providedover the insulation as sho!n in .ig"*iv+ Bedding" 1ver the metallic sheath is applied a layer of

bedding !hich consists of a )brous material like ute or hessiantape" #he purpose of bedding is to protect the metallic sheathagainst corrosion and from mechanical inury due to armouring"*v+ Armouring" 1ver the bedding, armouring is provided !hichconsists of one or t!o layers of galvanised steel !ire or steel

tape"$ts purpose is to protect the cable from mechanical inury!hile laying it and during the course of handling"*vi+ Serving" $n order to protect armouring from atmosphericconditions, a layer of )brous material *like ute+ similar tobedding is provided over the armouring" #his is kno!n asserving"2Bedding, armouring and serving are only applied to the cables


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Electrical Characteristics ofCables

Electrical Stress in Single&Core Cables

Capacitance of Single Core CablesCharging Current$nsulation resistance of Single Core CablesDielectric Po!er .actor 3 dielectric 4osses

5eating of Cables% Core loss6 Dielectric 4oss and$ntersheath 4oss


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Single&core 4o! #ension Cable

A cable may have one or more than one core depending

upon the type of service for !hich it is intended" $t may be*i) single&core *ii) two-core (iii) three-core (iv) four-coreetc.

.ig" sho!s the constructional details of a single&core lo!tension cable" #he cable has ordinary constructionbecause the stresses developed in the cable for lo!

voltages *upto ;;88 9+ are generally small"

$t consists of one circular core of tinned stranded copper*or aluminium+ insulated by layers of impregnated paper"

#he insulation is surrounded by a lead sheath !hich

prevents the entry of moisture into the inner parts"

$n order to protect the lead sheath from corrosion, anoverall serving of compounded )brous material *ute etc"+is provided"


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Cables for /&PhaseService#o deliver /&phase po!er either three&core cable or

three single core cables may be used" .or voltagesup to ;; k9, /&core cable *i"e", multi&coreconstruction+ is preferred due to economic reasons".or voltages beyond ;; k9, /&core&cables become

too large and bulky and, therefore, single&corecables are used"

#he follo!ing types of cables are generally used for/&phase service %

7" Belted cables < upto 77 k9:" Screened cables < from :: k9 to ;; k9/" Pressure cables < beyond ;; k9"


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Belted Cables.ig" sho!s the constructionaldetails of a /&core belted cable"

#he cores are insulated from eachother by layers of impregnatedpaper" Another layer ofimpregnated paper tape, calledpaper belt is !ound round the

grouped insulated cores" #he gapbet!een the insulated cores is )lled!ith )brous insulating material*uteetc"+ so as to give circular cross&

section to the cable"

#he cores are generally stranded and may be of noncircularshape to make better use of available space"#he belt is covered

!ith lead sheath to protect the cable against ingress of moistureand mechanical inury"#he lead sheath is covered !ith one ormore layers of armouring !ith an outer serving *not sho!n inthe )gure+"


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An additional conducting belt *copper !oven fabric

tape+ is !rapped round the three cores"#he cable has no insulating belt but lead sheath,bedding, armouring and serving follo! as usual" $t iseasy to see that each core screen is in electricalcontact !ith the conducting belt and the lead sheath"

As all the four screens */ core screens and oneconducting belt+ and the lead sheath are at earthpotential, therefore, the electrical stresses are purelyradial and conse(uently dielectric losses are reduced"

AdvantagesPerforations in the metallic screens assist in thecomplete impregnation of the cable !ith the compoundand thus the possibility of air pockets or voids *vacuousspaces+ in the dielectric is eliminated"0etallic screens increase the heat dissipating po!er

Screened cablescontd"


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Screened cablescontd"*ii+ S"4" type cables$t is basically 5&type cable butthe screen round each coreinsulation is covered by its o!nlead sheath" #here is no overalllead sheath but only armouring

and serving are provided"AdvantagesSeparate sheaths minimise thepossibility of core&to&core

breakdo!n" Bending of cables becomeseasy due to the elimination ofoverall lead sheath"Disadvantage

#hree lead sheaths of S"4"


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Pressure Cables

>hen the operating voltages are greater than ;; k9 and up to:/8 k9, pressure cables are used" $n such cables, voids are

eliminated by increasing the pressure of compound and for thisreason they are called pressure cables"#!o types of pressure cables vi' oil&)lled cables and gaspressure cables are commonly used"*i+ 1il&)lled cables

$n such types of cables, channels or ducts are provided in thecable for oil circulation" #he oil under pressure *it is the same oilused for impregnation+ is kept constantly supplied to thechannel by means of e-ternal reservoirs placed at suitabledistances *say @88 m+ along the route of the cable"

1il under pressure compresses the layers of paper insulation andis forced into any voids that may have formed bet!een thelayers"

1il&)lled cables are of three types vi'", single&core conductorchannel, single&core sheath channel and three&core )ller&space


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#he oil channel is formed at the centre by stranding the

conductor !ire around a hollo! cylindrical steel spiral tape" #heoil under pressure is supplied to the channel by means of e-ternalreservoir"

#he disadvantage of this type of cable is that the channel is atthe middle of the cable and is at full voltage !"r"t" earth, so that avery complicated system of oints is necessary"

Pressure Cables&1il.illed


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.ig" sho!s the constructional details of a single coresheath channel oil&)lled cable" $n this type of cable,the conductor is solid similar to that of solid cableand is paper insulated" 5o!ever, oil ducts areprovided in the metallic sheath as sho!n"



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$n the /&core oil&)ller cable sho!n in .ig" , the oil ducts arelocated in the )ller spaces" #hese channels are composed of

perforated metal&ribbon tubing and are at earth potential"

#he oil&)lled cables have three principal advantages".ormation of voids and ionisation are avoided" Allo!able temperature range and dielectric strength areincreased" $f there is leakage, the defect in the lead sheath is at once

indicated and the possibility of earth faults is decreased"Disadvantages are the high initial cost and complicated system



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C bl


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as Pressure Cablescontd".ig" sho!s the section of e-ternal pressurecable designed by 5ochstadter, 9ogal and

Bo!den"#he construction of the cable is oftriangular shape and thickness of leadsheath is @ that of solid cable" #hetriangular section reduces the !eight and

gives lo! thermal resistance but the mainreason for triangular shape is that the leadsheath acts as a pressure membrane" #hesheath is protected by a thin metal tape"

#he cable is laid in a gas&tight steel pipe" #he pipe is )lled !ithdry nitrogen gas at 7: to 7@ atmospheres" #he gas pressureproduces radial compression and closes the voids that may have

formed bet!een the layers of paper insulation"AdvantagesSuch cables can carry more load current and operate at highervoltages than a normal cable" 0oreover, maintenance cost is small and the nitrogen gas

helps in (uenching any ame"Disadvantage% the overall cost is very high"


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4aying of Underground Cables

Before laying cable under the ground, its route

should be surveyed 3 selected" #he position of !atermains or drains etc" Should be ascertained" Cables!hich are to be buried under ground must possessfollo!ing properties%

A+ moisture of soil should not enter the core ofcable"

B+ it must possess high insulation resistance"

C+ it should not be costly"

D+it should be su=ciently e-ible"E+ it should not be bulky"

.+ it should be able to !ithstand heat produced dueto o! of current"

+ it should not be capable of being damaged !hilela in in the round"


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#he reliability of underground cable net!ork

depends to a considerable e-tent upon theproper laying and attachment of )ttings i"e",cable end bo-es, oints, branch connectors etc"#here are three main methods of laying

underground cables % direct laying

dra!&in&system

solid&system

4aying of Underground Cables


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Direct 4aying$n direct laying method, thecables !ith steel tape or !irearmouring are laid directly asthey a?ord e-cellent protectionfrom mechanical inury" #hismethod of the laying

underground cables is simpleand cheap and in much use" $nthis method of laying, a trenchabout 7"@ m deep and @cm

!ide is dug through out theroute of the cable" #he trench iscovered !ith a layer of )nesand and the cable is laid overthis sand bed" #he purpose of

sand is to prevent the entry of


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After the laying of the cable in the trench, it is covered!ith another layer of sand of about 78 cm thickness,

care being taken that no sharp stone should come indirect contact !ith the cable"

#hen a single layer of bricks or tiles is laid to protectthe cable from protected by !ooden planks or

concrete slab">hen more than one cable is to be laid in the sametrench, hori'ontal or vertical intera-ial spacing at least/8cm is provided in order to reduce the e?ect of

mutual heating and also to ensure that fault occurringin any one cable does not damage the adacent cable"

#he cables !hich are to be laid by this !ay must haveserving of bituminised paper and hessian tape, so as

to provide protection against corrosion andelectrolysis"

Direct 4aying contd""


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Advantages*i+ $t is a simple and less costly method"

*ii+ $t gives the best conditions for dissipating the heatgenerated in the cables"*iii+ $t is a clean and safe method as the cable is invisibleand free from e-ternal disturbances"Disadvantages*i+ #he e-tension of load is possible only by a completelyne! e-cavation !hich may cost as much as the original!ork"*ii+ #he alterations in the cable net!ok cannot be made

easily"*iii+ #he maintenance cost is very high"*iv+ 4ocalisation of fault is di=cult"*v+ $t cannot be used in congested areas !heree-cavation is e-pensive and inconvenient"

#his method of laying cables is used in open areas !here

Direct 4aying contd""


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Dra!&in System

#his method of cable laying issuitable for congested areas!here e-cavation is e-pensive andinconvenient, for once theconduits have been laid, repairs oralterations can be made !ithout

opening the ground"$n this method, a line of conduitsor ducts are of the gla'edstone!are cement or concrete"

After laying conduits or ducts, the cables are put intothe position from man&holes or brick pits spaced atregular intervals".igure sho!s section through four !ay undergroundduct line" #hree of the ducts carry transmission cables

and the fourth duct carries relay protection


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Advantages*i+ epairs, alterations or additions to the cable net!ork can bemade !ithout opening the ground"*ii+ As the cables are not armoured, therefore, oints becomesimpler and maintenance cost is reduced considerably"*iii+ #here are very less chances of fault occurrence due tostrong mechanical protection provided by the system"

Disadvantages*i+ #he initial cost is very high"*ii+ #he current carrying capacity of the cables is reduced dueto the close grouping of cables and unfavourable conditions fordissipation of heat"

#his method is generally used for short length cable routessuch as in !orkshops, road crossings !here fre(uent digging iscostlier or impossible"

Dra!&in System contd"


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Solid System

$n this method of laying, the cable is laid in openpipes or troughs dug out in earth along the cableroute" #he troughing is of cast iron, stone!are,asphalt or treated !ood" After the cable is laid inposition, the troughing is )lled !ith a bituminous orasphaltic compound and covered over"

Cables laid in this manner are usually plain leadcovered because troughing a?ords good mechanicalprotection"Disadvantages

*i+ $t is more e-pensive than direct laid system"*ii+ $t re(uires skilled labour and favourable !eatherconditions"*iii+ Due to poor heat dissipation facilities, the currentcarrying capacity of the cable is reduced"

Comparison bet!een Underground 3


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Comparison bet!een Underground 31verhead System

Comparison Bet!een Underground 3


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Comparison Bet!een Underground 31verhead System Cont""


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