31506636 underground cables

8/3/2019 31506636 Underground Cables

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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.

5


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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.

31506636 underground cables

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