Transcript Underground Cables

ATMIYA INSTITUTE OF
TECHNOLOGY & SCIENCE
Underground Cables
Presented By:
Rathod Vedant(130030109096)
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Introduction
 Since the loads having the trends towards growing density. This
requires the better appearance, rugged construction, greater
service reliability and increased safety. An underground cable
essentially consists of one or more conductors covered with
suitable insulation and surrounded by a protecting cover. The
interference from external disturbances like storms, lightening,
ice, trees etc. should be reduced to achieve trouble free service.
The cables may be buried directly in the ground, or may be
installed in ducts buried in the ground.
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Introduction
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Advantages & Disadvantages
Advantages
 Better general appearance
 Less liable to damage through storms or lighting
 Low maintenance cost
 Less chances of faults
 Small voltage drops
Disadvantages
 The major drawback is that they have greater installation cost and
introduce insulation problems at high voltages compared with
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equivalent overhead system.
Construction of Cables
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Punjab EDUSAT Socity
Construction of Cables
 Core or Conductor
A cable may have one or more than one core depending upon the
type of service for which it is intended. The conductor could be of
aluminum or copper and is stranded in order to provide flexibility to
the cable.
 Insulation
The core is provided with suitable thickness of insulation, depending
upon the voltage to be withstood by the cable.
The commonly used material for insulation are impregnated paper,
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varnished cambric or rubber mineral compound.
Construction of Cables
 Metallic Sheath
A metallic sheath of lead or aluminum is provided over the
insulation to protect the cable from moisture, gases or
others damaging liquids
 Bedding
Bedding is provided to protect the metallic sheath from
corrosion and from mechanical damage due to armoring. It
is a fibrous material like jute or hessian tape.
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Construction of Cables
 Armouring
Its purpose is to protect the cable from mechanical injury
while laying it or during the course of handling. It consists
of one or two layers of galvanized steel wire or steel tape.
 Serving
To protect armouring from atmospheric conditions, a layer
of fibrous material is provided.
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Insulating Materials for Cables
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Properties of Insulating Material
 High resistivity.
 High dielectric strength.
 Low water absorption.
 Non – inflammable.
 Chemical stability.
 High mechanical strength.
 Capability to with stand high rupturing voltage.
 High tensile strength.
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Insulating Materials for Cables
• Vulcanized India Rubber
It can be obtained from mixing pure rubber with mineral compounds i-e zinc
oxide, red lead and sulphur and heated upto 150 C.
It has greater mechanical strength, durability and wear resistant property.
The sulphur reacts quickly with copper so tinned copper conductors are used.
It is suitable for low and moderate voltage cables.
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CLSSIFICATION OF CABLES
 Low tension (L.T) ----- up to 1000V
 High tension (H.T) ----- up to 11, 000V
 Super tension (S.T) ---- from 22KV to 33KV
 Extra high tension (E.H.T) cables --- from 33KV to
66KV
 Extra super voltage cables ------beyond 132KV
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3- Core Cables
Belted Cables
 In these cables the conductors are wrapped with oil impregnated
paper, and then cores are assembled with filler material. The
assembly is enclosed by paper insulating belt.
 These can be used for voltages up to 11KV or in some cases can be
used up to 22KV.
 High voltages beyond 22KV, the tangential stresses becomes an
important consideration.
 As the insulation resistance of paper is quite small along the layer,
therefore tangential stress set up, hence, leakage current along the
layer of the paper insulation.
 This leakage current causes local heating, resulting breaking of
insulation at any moment
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3-core belted Cable
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3- Core Cables
 Screened Cables
 These can be used up to 33kv but in certain cases can be
extended up to 66kv.
 These are mainly of two types
 H-type and
 S.L type cables
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3- Core Cables
H-TYPE Cables:
 Designed by H. Hochstadter.
 Each core is insulated by layer of impregnated paper.
 The insulation on each core is covered with a metallic screen
which is usually of perforated aluminum foil.
 The cores are laid in such a way that metallic screen make
contact with one another.
 Basic advantage of H-TYPE is that the perforation in the
metallic screen assists in the complete impregnation of the
cable with the compound and thus the possibility of air
pockets or voids in the dielectric is eliminated.
 The metallic screen increase the heat dissipation power of the
cable.
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3- Core Cables (H-Type)
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3- Core Cables
S.L - Type: (Separate Lead)
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Each core insulation is covered by its own lead sheath.
It has two main advantages, firstly the separate sheath minimize
the possibility of core-to-core breakdown. Secondly the,
bending of cables become easy due to the elimination of over
all sheath.
The disadvantage is that the lead sheaths of S.L is much thinner
as compared to H-Type cables, therefore for greater care is
required in manufacturing.
3- Core Cables (S.L. Type)
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3- Core Cables
 Pressurized Type Cables
In these cables, pressure is maintained above atmosphere
either by oil or by gas.
 Gas pressure cables are used up to 275KV.
 Oil filled cables are used up to 500KV.

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3- Core Cables
 Oil Filled Cables
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a.
b.
c.
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Low viscosity oil is kept under pressure and fills the voids
in oil impregnated paper under all conditions of varying
load.
There are three main types of oil filled cables
Self-contained circular type
Self-contained flat type
PipeType cables
3- Core Cables (Oil filled)
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Laying of Underground Cables
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a.
b.
c.
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The reliability of underground cable network depends to a
considerable extent upon proper laying.
There are three main methods of Laying underground
cables
Direct Laying
Draw in system
Solid system
Direct Laying
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This method is cheap and simple and is most likely to be
used in practice.
A trench of about 1.5 meters deep and 45 cm wide is dug.
A cable is been laid inside the trench and is covered with
concrete material or bricks in order to protect it from
mechanical injury.
This gives the best heat dissipating conditions beneath the
earth.
It is clean and safe method
Direct Laying
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Disadvantages of Direct Laying
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Localization of fault is difficult
It can be costlier in congested areas where
excavation is expensive and inconvenient.
The maintenance cost is high
Draw in System
 In this conduit or duct of concrete is laid in ground with
main holes at suitable positions along the cable route.
 The cables are then pulled into positions from main holes.
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Advantages of Draw in System
 It is very high initial cost
 Heat dissipation conditions are not good
 This method is suitable for congested areas where excavation
is expensive and inconvenient
 This is generally used for short lengths cable route such as in
workshops, road crossings where frequent digging is costlier
and impossible
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Solid System
 In this system the cable is laid in open pipes or troughs dug out in
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earth along the cable route.
The troughing is of cast iron or treated wood
Troughing is filled with a bituminous after cables is laid.
It provides good mechanical strength
It has poor heat dissipation conditions
It requires skilled labour and favorable weather conditions
It is very much expensive system
Solid System
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Grading of Cables
• Since the stresses are maximum at surface of the conductor
or inner most part of the dielectric.
• The stress goes on decreasing as outer most layer is reached.
• Since the process of achieving the uniform electrostatic
stresses on the dielectric of cables is known as Grading of
cables
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Grading of Cables
• The unequal distribution of stresses is undesirable because,
 if dielectric is chosen according to maximum stress the
thickness of cable increases or either this may lead to
breakdown of insulation.
• The following are the two main methods of grading
 Capacitance grading
 Inter sheath grading
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EARTH FAULTS
 When the conductor of a cable comes in contact with earth,
it is called earth fault or ground fault.
 To identify this fault, one terminal of the megger is
connected to the conductor and the other terminal
connected to earth.
 If the megger indicates zero reading, it means the conductor
is earthed. The same procedure is repeated for other
conductors of the cable.
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Summary of Costs: Overhead vs.
Underground
 Transmission: Underground may be 4-20 times Overhead.
 Sub transmission: Underground may be 4-20 times Overhead
 Distribution: Underground may be 2-10 times Overhead
 New underground may be cheaper than overhead in special
conditions and costs vary greatly from utility to utility and
place to place.
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THANKS
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