UG Cables.pptx

8/9/2019 UG Cables.pptx

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Underground

Cables Presented By:

ADNAN

Lecturer Deptt. Of Electrical Engg NED UET Karachi

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IntrductinAn underground cable essentially consists of

one or more conductors covered with suitable

insulation and surrounded by a protectingcover.

The cables may be buried directly in the

ground, or may be installed in ducts buried inthe ground.

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Most common Conductors used areCopper:

The principal electrical conductor due to its excellent

conductivity and reasonable cost.

Aluminum:

61% conductivity as compared to copper and 0.3027 times

the weight o copper. !sed in power cables" overhead

#$&'(.


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Comparison

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A vantages "#isadvantages

Ad!antages

$etter general appearance

%ess liable to damage through storms or lighting

%ow maintenance cost

%ess chances of faults

&mall voltage drops

Disad!antagesThe ma'or drawbac( is that they have greater installation

cost and introduce insulation problems at high voltages

compared with e)uivalent overhead system.

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CABLE STRUCTURE

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Construction of cableParts of a power cable:

$ cable is composed o the ollowing parts)

Cores:

$ll cables either have a central core #conductor( or a number o cores made o strandso opper or $luminum conductors. onductors are stranded in order to reduce the

s*in eect.

Insulation:

+t is provided to insulate the conductors rom each other and rom the outsideperiphery. The common insulating materials are ,oly -inyl hloride #,-( and /,.

Metallic Sheath:

etallic &heath protects the cable against the entry o moisture. +t is made o lead"

some alloy o lead or $luminum.


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CABLE STRUCTURE

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Construction of cableBein!:

+n order to protect the metallic sheath rom inury" bedding is wound over it. +t

consists o paper tape compounded with a ibrous material.

Armorin!:

+t consists o one or two layers o galvanied steel wires or two layers o steel

tape" to avoid the mechanical inury. $rmoring provides mechanical strength

to the cable.

Ser"in!:

$ layer o ibrous material" used to protect the armoring


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Desired Prperties f

Insulating "aterial/igh resistivity.

/igh dielectric strength.

%ow water absorption.

0on inammable.

Chemical stability.

/igh mechanical strength.

/igh tensile strength and plasticity.

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Prperties f Insulating

"aterial

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Insulating "aterials fr #a$les%u$$er

&ulcani'ed India %u$$er

I(pregnated Paper

&arnished #a($ric

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Insulating "aterials fr #a$lesPly!inyl chlride )P*This material has good dielectric strength, high insulation

resistance and high melting temperatures.

4t is inert to o5ygen and almost inert to many al(alis and

acids.

+LPE #a$les )#rss Lin,ed Ply-ethene*

This material gives good insulating properties

4t is light in weight and high mechanical strength, low water

absorption.

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6%78 cable

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#LI/I#ATION O/

#ABLEL0 tensin )L.T* ----- up t 1222&

3igh tensin )3.T* ----- up t 114 222&

uper tensin ).T* ---- fr( 55K& t 66K&

E7tra high tensin )E.3.T* ca$les --- fr(

66K& t 88K&

E7tra super !ltage ca$les ------$eynd 165K&

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6- #re #a$lesPressuri'ed Type #a$les

4n these cables, pressure is maintainedabove atmosphere either by oil or by gas.

9as pressure cables are used up to 2*:;.


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6- #re #a$les )9as

pressure*

7un'ab 8#U&AT &ocity 1


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9as Pressure #a$les

7un'ab 8#U&AT &ocity 1-


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9as Insulated #a$les )9I#*

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Laying f

Undergrund #a$lesThe reliability of

underground cablenetwor( depends to aconsiderable e5tent upon

proper laying.

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Direct Laying This method is cheap and simple and is

most li(ely to be used in practice.

A trench of about 1.* meters deep and !*cm wide is dug.

A cable is been laid inside the trench and iscovered with concrete material or bric(s in

order to protect it from mechanical in'ury. This gives the best heat dissipating

conditions beneath the earth.

4t is clean and safe method

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Direct Laying

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#isadvantages of #irect

%aying %ocali>ation of fault is di?cult

4t can be costlier in congested areas where

e5cavation is e5pensive and inconvenient.

The maintenance cost is high

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9rading f #a$les &ince the stresses are ma5imum at surface of

the conductor or inner most part of thedielectric.

The stress goes on decreasing as outer mostlayer is reached.

&ince the process of achieving the uniformelectrostatic stresses on the dielectric ofcables is (nown as 9rading of cables

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TPE O/ #ABLE /AULT

Cables are generally laid in the ground or inducts in the underground distribution system.@or this reason, there are little chances of faults

in underground cables. /owever, if a fault doesoccur it is di?cult to locate and repair the faultbecause conductors are not visible.0evertheless, the following are the faults most

li(ely to occur in underground cables1 open circuit fault2 short circuit faultearth fault

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OPEN #I%#UIT /AULT

4hen there is a brea* in the conductor o a cable" it is

called open circuit ault.

The open circuit ault can be chec*ed by megger. 5or this

purpose" the three conductors o the 3core cable at the ar

end are shorted and earthed.

The resistance between each conductor and earth is

measured by a megger and it will indicate ero resistancein the circuit o the conductor that is not bro*en.

owever" i the conductor is bro*en" the megger will

indicate ininite resistance in its circuit

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3O%T #I%#UIT /AULT

Bhen two conductors of a multicore cable comein electrical contact with each other due toinsulation failure, it is called a short circuit fault.

Again, we can see( the help of a megger to chec(this fault.

@or this purpose the two terminals of the meggerare connected to any two conductors.

4f the megger gives >ero reading, it indicates shortcircuit fault between these conductors.

The same steps is repeated for other conductorsta(ing two a time.

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EA%T3 /AULTBhen the conductor of a cable comes in

contact with earth, it is called earth fault orground fault.

To identify this fault, one terminal of themegger is connected to the conductor and theother terminal connected to earth.

4f the megger indicates >ero reading, it meansthe conductor is earthed. The same procedureis repeated for other conductors of the cable.

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Summary of Costs: Overhead vs. UndergroundTransmissionD Underground may be 2! times

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UG Cables.pptx