10022014091851 underground cables (done)

8/9/2019 10022014091851 Underground Cables (Done)

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Seminar guide Head of theDepartment

ABSTRAC T

Underground cables are the electric power transmission cables. Because of their

reliability on transmitting, they used in congested urban areas and in thick populationareas, where overhead transmission is dangerous. Underground cables have low

maintenance cost, less chances of faults, smaller voltage drop. In recent

improvements in the design and manufacture have led to development of cables

suitable for use at high voltage

CONTENT

Introduction

Construction

Insulating materials used

Classification

Lay out of cables

Grading of cables

dvantages

!isadvantages

Conclusion

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INTRODUCTION

"lectric power can be transmitted or distributed either by overhead system or by

underground cables. #he underground cables have several advantages then overhead system, so

we use underground cables.



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CONSTRUCTION

CORE/CONDUCTORS: -

cable may have one or three core $Conductor% depending upon the type of service for

which it is intended. &or instance, the three conductor cable is used for '(phase service.

#he conductors are made of tinned copper or aluminium and are usually stranded in

order to provide fle)ibility to the cable.



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INSULATION: -

"ach 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. #he commonly used

materials for insulation are impregnated paper, varnished cambric or rubber material

compound.

METALLIC SHEATH: -

In order to protect the cable from moisture, gases or other damaging li*uids in the soil +

atmosphere, a metallic sheath of lead or aluminium is provided over the insulation.

ARMOURING: -

rmouring is provided which consists of one or two layers of galvanised steel wire or

steel tape. It purpose is to protect the cable from mechanical inury while laying it and

during the course of handling. rmouring may not be done in the case of some cables.

-"/I0G1 (

In order to protect armouring from atmospheric conditions, a layer of fibrous material is

provided over the armouring. #his is known as serving. It may not be out of place to

mention here that armouring and serving are only applied to the cables for the protection

of conductor insulation and to protect the metallic sheath from mechanical inury.

INSULATING MATERIALS USED



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UBB"1 (

ubber may be obtained from milky sap of tropical trees or it may be produced from oil

products. It has relative permittivity varying between 2 and ', dielectric strength is about

'3kv4mm and resistivity of insulation is 53657 8 cm. lthough pure rubber has

reasonable high insulating properties, is suffers from some maor drawbacks vi9., readily

absorbs moisture, ma)imum safe temperature is low $about ' C%, soft and liable to8

damage due to rough handling and ages when e)posed to light. :ure rubber cannot be

used as insulating material.

I;:"G0#"! ::"1 (

It consists of chemically pulped paper made from wood chippings and impregnated with

some compound such as paraffinic or napthenic material. #his type of insulation has

almost superseded the rubber insulation. It is because it has the advantages of low cost,

low capacitance, high dielectric strength and high insulation resistance. #he only

disadvantage is that paper is hygroscopic and even if it is impregnated with suitable

compound, it absorbs moisture and thus lowers the insulation resistance of the cable. &or

this reason, paper insulated cables are always provided with some protective covering

and are never left unsealed. If it is re*uired to be left unused on the site during laying, its

ends are temporarily covered with wa) or tar. It absorbs moisture so it provided with

protective covering + does not left unsealed. It was high dielectric strength, high

insulation resistance.

/0I-


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Cables classified in two ways

% Based on /oltage

CC@!I0G #@ /@L#G" I- !I/I!"! I0#@ &@LL@AI0G-1(

Low tension cables up to 5kv


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THREE CORE CABLE

Underground cables are generally re*uired to deliver ' phase power. &or the purpose, either

three(core cable or three single core cables may be used. &or voltages up to kv, '(core cable is

preferred due to economic reasons.


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Generally they are ' types of three core cable.

5. Belted cables up to 55kv

2. -creened cables from 22kv to kv

'. :ressure cables ( beyond kv.

$5% Belted cables1 (

#hese cables are used for voltages up to 55kv but in e)traordinary cases, their use

may be e)tended up to 22kv. #he cores are insulated from each other by layers of

impregnated paper. nother layer of impregnated paper tape, called paper belt is

wound round the grouped insulated cores. #he gap between the insulated cores is

filled with fibrous insulating material so as to give circular cross(section to the cable.

#he belt is covered with lead sheath to protect the cable against ingress of moisture



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and mechanical inury. #he lead sheath is covered with one or more layers of

armouring with an outer serving. #he belted type construction is suitable only for

low and medium voltages as the electrostatic stresses developed in the cables for

these voltages are more or less radial i.e., across the insulation. #hese stresses act

along the layers of paper insulation. s the insulation resistance of paper is *uite

small along the layers, therefore, tangential stresses set up leakage current along the

layers of paper insulation. #he leakage current causes local heating, resulting in the

risk of breakdown of insulation at any moment. In order to overcome this difficulty,

screened cables are used where leakage currents are conducted to earth through

metallic screens.

$2% -creened cables1 (

#hese cables are meant for use up to ''kv, but in particular cases their use may

be e)tended to operating voltage up to ''kv. #wo principal types of screened

cables are


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$'% :ressure cables1 (

&or voltages beyond kv, solid type cables are unreliable because there is a danger

of breakdown of insulation due to the presence of voids. Ahen the operating voltages

are greater than kv, pressure cables are used. In such cables, voids are eliminated

by increasing the pressure of compound and for this reason they are called pressure

cables.



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LAYOUT OF CABLES

#he reliability of underground cable network depends to a considerable e)tent upon

the proper laying and attachment of fittings i.e., cable end bo)es, oints, branch

connectors etc. #here are three main methods of laying underground cables,

$5% !irect laying,

$2% !raw(in system

$'% -olid system

$5% !irect laying1 (

#his method of laying underground cables is simple and cheap and is much

favoured in modern practice. In this method, a trench of about 5.> metres

deep and => cm wide is dug. #he trench is covered with a layer of fine sand

and the cable is laid over this sand bed. #he sand prevents the entry of

moisture from the ground and thus protects the cable from decay. fter the

cable has been laid in the trench, it is covered with another layer of sand of

about 53 cm thickness. #he trench is then covered with bricks and other

materials in order to protect the cable from mechanical inury. Ahen more

than one cable is to be laid in the same trench, a hori9ontal or vertical

intera)ial spacing of at least '3cm 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 adacent 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 electrolysis.

$2% !raw in system1 (

In this method, duct of gla9ed stone or cast iron or concrete are laid in the

ground with manholes at suitable position along the cable route. #he cables

are then pulled into position from manholes. -ection through four way

underground duct line. #hree of the ducts carry transmission cables and the



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fourth duct carries relay protection connection, pilot wires. Care must be

taken

that where the duct line changes directionE depths, dips and offsets be made

with a very long radius or it will be difficult to pull a large cable between the

manholes. #he distance between the manholes should not be too long so as to

simplify the pulling in of the cables. #he cables to be laid in this way need

not be armoured but must be provided with serving of hessian and ute in

order to protect them when being pulled into the ducts.

$'% -olid system1 (

In this method of laying, the cable is laid in open pipes or troughs dug out in

earth along the cable route. #he troughing is of cast iron, stoneware, asphalt

or treated wood. fter 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.



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GRADING OF CABLES

#he process of achieving uniform electrostatic stress in the dielectric of

cables is known as grading of cables.

It has already been shown that electrostatic stress in a single core cable has a

ma)imum value $g ma)% at the conductor surface and goes on decreasing as

we move towards the sheath. #he ma)imum voltage that can be safely

applied to a cable depends upon g ma) i.e., electrostatic stress at the

conductor surface. &or safe working of a cable having homogeneous

dielectric, the strength of dielectric must be more then g ma). If a dielectric

of high strength is used for a cable, it is useful only near the conductor where

stress is ma)imum. But as we move away from the conductor, the

electrostatic stress decreases, so the dielectric will be unnecessary

overstrong. #he une*ual stress distribution in a cable is undesirable for two

reasons. &irstly, insulation of greater thickness is re*uired which increases

the cable si9e. -econdly, it may lead to the breakdown of insulation. In order

to overcome above disadvantages, it is necessary to have a uniform stress

distribution in cables. #his can be achieved by distributing the stress in such

a way that its value is increased in the outer layers of dielectric. #his is

known as grading of cables.



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ADVANTAGES

Less liable to damage through storms or lightning

Low maintenance cost

Less chances of faults

-maller voltage drop



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DISADVANTAGES

Installation cost is more

Introduce insulation problems at high voltage as compared to overhead system



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CONCLUSION

Underground cables are the ultimate solution of over head transmission lines. In recent past, the

electric transmission system is completely destroyed in thunder storms and cyclones. Using

underground cable those destruction are eliminated. But it has high initial cost for installation.



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REFERENCE

:ower system analysis + design by B.. Gupta, -. Chand publications, ' rdeditions

:ower system by /.F. ;ehta


10022014091851 underground cables (done)

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