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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
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