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Underground PowerTransmission
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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 conductorscovered 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 inducts buried in the ground.
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Advantages
The underground cables have several advantages such as,
Better general appearance
Less liable to damage through storms or lighting
Low maintenance cost
Less chances of faults
Small voltage drops
Dis-advantage
The major drawback is that they have greater installation cost and introduce
insulation problems at high voltages compared with equivalent overheadsystem.
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Overhead vs. Underground
Transmission: Underground may be 4-20 times Overhead.
Sub transmission: Underground may be 4-20 timesOverhead.
Distribution: Underground may be 2-10 times Overhead.
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Construction of Cables
Core or Conductor
A cable may have one or more than onecore depending upon the type of service forwhich it is intended. The conductor couldbe of aluminium or copper and is strandedin order to provide flexibility to the cable.
Insulation
The core is provided with suitable thickness
of insulation, depending upon the voltage tobe withstood by the cable.
The commonly used material for insulationare impregnated paper, varnished cambricor rubber mineral compound.
Metallic SheathA metallic sheath of lead or aluminium isprovided over the insulation to protect thecable from moisture, gases or othesdamaging liquids
Core
Belted paper
Lead sheath
Bedding
Single wirearmoring
Overall Serving
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BeddingBedding is provided to protect the metallic sheath from corrosion andfrom mechanical damage due to armoring. It is a fibrous material like
jute or hessian tape.
ArmouringIts purpose is to protect the cable from mechanical injury while laying itor during the course of handling. It consists of one or two layers ofgalvanized steel wire or steel tape.
ServingTo protect armouring from atmospheric conditions, a layer of fibrousmaterial is provided.
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Properties of Insulating Material
The insulating materials used in cables should have the following properties
High resistivity.
High dielectric strength.
Low thermal co-efficient.
Low water absorption.
Low permittivity.
Non inflammable.
Chemical stability.
High mechanical strength.
High viscosity at impregnation temperature.
Capability to with stand high rupturing voltage.
High tensile strength and plasticity.
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Insulating Materials for Cables
Rubber
It can be obtained from milky sap of tropical trees or from oil products.
It has the dielectric strength of 30 KV/mm.Insulation resistivity of 10 exp 17 ohm.cm
Relative permittivity varying between 2 and 3.
They readily absorbs moisture, soft and liable to damage due to rough handling and ages
when exposed to light.
Maximum safe temperature is very low about 38 C
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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Impregnated Paper
This material has superseded the rubber, consists of chemically pulped paper
impregnated with napthenic and paraffinic materials.
It has low cost, low capacitance, high dielectric strength and high insulation
resistance.
The only disadvantage is the paper is hygroscopic, for this reason paper insulation is
always provided protective covering.
Varnished Cambric
This is simply the cotton cloth impregnated and coated with varnish.
As the varnish cambric is also hygroscopic so need some protection.
Its dielectric strength is about 4KV / mm and permittivity is 2.5 to 3.8.
Polyvinyl chloride (PVC) This material has good dielectric strength, high insulation resistance and high
melting temperatures.
These have not so good mechanical properties as those of rubber.
It is inert to oxygen and almost inert to many alkalis and acids.
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XLPE Cables (Cross Linked Poly-ethene)
This material has temperature range beyond 250 300 C
This material gives good insulating properties
It is light in weight, small overall dimensions, low dielectric constant and high
mechanical strength, low water absorption.
These cables permit conductor temperature of 90 C and 250 C under normal and
short circuit conditions.
These cables are suitable up to voltages of 33 KV.
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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
A cable may have one or more than one core depending upon the type of
service for application. It may be,Single Core
Two Core
Three Core
Four Core
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3- Core Cables Solid Type Cables
1. 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 animportant consideration.
As the insulation resistance of paper is quite small along the layer,therefore tangential stress set up, hence, leakage current along thelayer of the paper insulation.
This leakage current causes local heating, resulting breaking ofinsulation at any moment.
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2. 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
a. 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 usuallyof 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 airpockets or voids in the dielectric is eliminated.
The metallic screen increase the heat dissipation power of the cable.
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b. S.L - Type:(Separate Lead)
Each core insulation is covered by its own lead sheath.
It has two main advantages, firstly the separate sheath minimize thepossibility 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 inmanufacturing.
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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.
Oil Filled Cables
Low viscosity oil is kept under pressure and fills the voids in oil impregnated paperunder all conditions of varying load.
There are three main types of oil filled cables
a. Self-contined circular type
b. Self-contained flat type
c. Pipe Type cables
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Pipe Type Cable
Sheath Channel Oil Filled 3-Core Oil filler Cable
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Advantages of Oil Filled Cables
Oil filled cables have the following advantages over solid cables
Greater operating dielectric stresses
Greater working temperature and current carrying capacity Better impregnation
Impregnation is possible after sheath
No void formation
Smaller size of cable due to reduced dielectric thickness
Defect can easily be detected by oil leakage
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Gas Pressure Cables
In these cables an inert gas like nitrogen is used to exert pressure on paper dielectric to preventvoid formation.
These are also termed as Compression cables They insulated cores similar to solid type
The cable is inserted in a pressure vessel which may be a rigid steel pipe, commonly knownas pipe line compression cable.
The nitrogen gas is filled in vessel at nominal pressure of 1.38 * 10 exp 6 N/ square meterwith a maximum pressure of 1.725 * 10 exp 6 N/ square meter.
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Compressed Gas Insulated Cables (GIC)
In GIC cables high pressure sulphur hexaflouride (SF6), fills the smallspaces in oil impregnated paper insulation and suppresses the ionization.
Most EHV and UHV lines insulated with sulphur hexaflouride (SF6) gas are
being used extensively for voltages above 132 KV up to 1200 KV. These cables are very popular for short lengths, river crossings and high
way crossings.
Advantages
Gas Insulated Cables have several advantages over oil filled cables,
Efficient heat transfer hence can carry more current.
Low dielectric loss and low capacitance
SF6 gas is non-toxic, chemically stable and non-inflamable.
Terminations of GIC cables are simpler and cheaper.
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Laying of Underground Cables
The reliability of underground cable network depends to a considerable extent uponproper laying.
There are three main methods of Laying underground cables
a. Direct Laying
b. Draw in system
c. Solid system
Direct Laying
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 inorder to protect it from mechanical injury.
This gives the best heat dissipating conditions beneath the earth.
It is clean and safe method
Disadvantages
Localization of fault is difficult
It can be costlier in congested areas where
excavation is expensive and inconvenient.
The maintenance cost is high.
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Draw in System
In this conduit or duct of concrete is laid in ground with main holes at suitablepositions along the cable route.
The cables are then pulled into positions from main holes.
It is very high initial cost
Heat dissipation conditions are not good
This method is suitable for congested areas where excavation is expensive andinconvenient
This is generally used for short lengths cable route such as in workshops, road
crossings where frequent digging is costlier and immposible
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Solid System
In this system the cable is laid in open pipes or troughs dug out in earthalong 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
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Grading of Cables
Since the stresses are maximum at surface of the conductor or inner mostpart of the dielectric.
The stress goes on decreasing as outer most layer is reached.
Since the process of achieving the uniform electrostatic stresses on thedielectric of cables is known as Grading of cables.
The unequal distribution of stresses is undesirable because,
if dielectric is chosen according to maximum stress the thickness of cableincreases 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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A power cable mainly consists of three components Conductor
Insulation /Dielectric
sheath
These are of high conductivity copper or aluminum wires
touched together to form a required shape.
Copper and aluminum conductors are used as conductor
materials.
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TYPES OF MATERIALS USED IN INSULATION.
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High resitivity.
High dielectric strength.
Low thermal co-efficient.
Low water absorption.
Low permittivity.
Noninflammable.
Chemical stability.
High mechanical strength.
High viscosity at impregnation temperature.
Capability to with stand high rupturing voltage.
High tensile strength and plasticity.
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One or three such paper insulated conductors are placed in a
circular form and group is covered with impregnated paper
again. Necessary filler is also covered.
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Cables are classified according to their insulation into following
main types:
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1. Low voltage
(L.T.) cable
(operating
Voltage up to 1
KV
3.Super
tension (S.T)
Cable
(operating
voltage
Up to 33 KV.) 5.Extra super
voltageCable
(operating
voltage
up to 132 KV.4. Extra super
tension
(E.H.T.) cable
(operating
Voltage up to66KV.
2. High
voltage (H.T)
Cable
(operating
voltageUp to 11 KV)
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TRENCHES
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Cables are generally laid in the ground or in ducts in the
underground distribution system. For this reason, there are
little chances of faults in underground cables. However, if a
fault does occur it is difficult to locate and repair the faultbecause conductors are not visible. Nevertheless, the following
are the faults most likely to occur in underground cables
1) open circuit fault
2) short circuit fault
3)earth fault
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When there is a break in the conductor of a cable, it is called open circuit fault. The open circuit fault can be checked by megger. For this purpose, the three
conductors of the 3-core cable at the far end are shorted and earthed.
The resistance between each conductor and earth is measured by a megger and it
will indicate zero resistance in the circuit of the conductor that is not broken.
However, if the conductor is broken, the megger will indicate infinite resistance in
its circuit.
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When two conductors of a multi-core cable come in electrical contact witheach other due to insulation failure, it is called a short circuit fault.
Again, we can seek the help of a megger to check this fault.
For this purpose the two terminals of the megger are connected to any twoconductors.
If the megger gives zero reading, it indicates short circuit fault betweenthese conductors.
The same steps is repeated for other conductors taking two a time.
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When the conductor of a cable comes in contact with earth, it is called earthfault or ground fault.
To identify this fault, one terminal of the megger is connected to theconductor and the other terminal connected to earth.
If the megger indicates zero reading, it means the conductor is earthed. Thesame procedure is repeated for other conductors of the cable.
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SF6 Filled Cable
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XLPE cable
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Gas pressure cables.
(A) External pressure cables.(B) Internal pressure cable.
(a) High pressure gas filled cable.
(b) Gas cushion cable.
(c) Impregnated pressure cable
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Oil filled cables.
(A) Single core oil filled cables used up to 132 KV.
(B) Three core oil filled cables used up to 66 KV.
EXTRA SUPER VOLTAGE CABLE:-
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The purpose of the external layer in insulated power cables is to provide
mechanical protection against the environment during the installation and
operation of the power cable.
Currently, materials commonly used as the external layer for extruded
power cables include PVC and polyethylene.
These materials are used for their ability to withstand the cable operating
temperature.