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8/10/2019 Roshan new.ppt

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INTRODUCTION TO UNDERGROUND

CABLESPRESENTED BY:

ROSHAN PRADHAN

C.V.RAMAN COLLEGE OF ENGINEERINGBBSR

C.V.RAMAN GROUP OF INSTITUTIONS. 1


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UNDERGROUND CABLES

PRESENTED BY:

ROSHAN PRADHAN

REGD NO - 1221227062

SEC-2, GROUP-C

SEM-7th,YEAR-4th

ELECTRICAL ENGINEERING

C.V.RAMAN GROUP OF INSTITUTIONS 2


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Introduction

Since the loads having the trends towards growingdensity. This requires the better appearance, ruggedconstruction, greater service reliability and

increased safety. An underground cable essentiallyconsists of one or more conductors covered withsuitable insulation and surrounded by a protectingcover. The interference from external disturbanceslike storms, lightening, ice, trees etc. should bereduced to achieve trouble free service. The cablesmay be buried directly in the ground, or may beinstalled in ducts buried in the ground.

C.V.RAMAN GROUP OF INSTITUTIONS3


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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

equivalent overhead system.



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Construction of Cables

Metallic SheathA metallic sheath of lead or aluminum isprovided over the insulation to protect the

cable from moisture, gases or othersdamaging liquids

Bedding

Bedding is provided to protect the metallicsheath from corrosion and from mechanicaldamage due to armoring. It is a fibrousmaterial like jute or hessian tape.



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Construction of Cables

Armouring

Its purpose is to protect the cable frommechanical injury while laying it or during thecourse of handling. It consists of one or twolayers of galvanized steel wire or steel tape.

Serving

To protect armouring from atmosphericconditions, a layer of fibrous material isprovided.



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Properties of Insulating

Material 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 CC.V.RAMAN GROUP OF INSTITUTIONS

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Insulating Materials for Cables

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.

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



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Extra High Tension Cable



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Low Tension Cable



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3- Core CablesBelted Cables

In these cables the conductors are wrapped with oil impregnatedpaper, and then cores are assembled with filler material. Theassembly 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 thelayer, 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 CablesH-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 screenwhich is usually of perforated aluminum foil.

The cores are laid in such a way that metallic screen makecontact with one another.

Basic advantage of H-TYPE is that the perforation in themetallic screen assists in the complete impregnation of thecable 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) Each core insulation is covered by its own lead

sheath.

It has two main advantages, firstly the separatesheath minimize the possibility of core-to-corebreakdown. Secondly the, bending of cablesbecome easy due to the elimination of over allsheath.

The disadvantage is that the lead sheaths of S.L ismuch thinner as compared to H-Type cables,therefore for greater care is required inmanufacturing.



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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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Laying of Underground Cables

The reliability of underground cable networkdepends to a considerable extent uponproper laying.

There are three main methods of Layingunderground cables

a. Direct Laying

b. Draw in system

c. Solid system



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Direct Laying

This method is cheap and simple and is mostlikely 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 conditionsbeneath the earth.

It is clean and safe method



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Direct Laying



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Disadvantages of Direct Laying

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 inground with main holes at suitable positionsalong the cable route.

The cables are then pulled into positions frommain 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 areaswhere excavation is expensive andinconvenient

This is generally used for short lengths cableroute such as in workshops, road crossingswhere frequent digging is costlier andimpossible



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Solid System

In this system the cable is laid in open pipes ortroughs dug out in 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 weatherconditions

It is very much expensive system



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Solid System



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Grading of Cables

Since the stresses are maximum at surface ofthe conductor or inner most part of thedielectric.

The stress goes on decreasing as outer mostlayer is reached.

Since the process of achieving the uniform

electrostatic stresses on the dielectric ofcables is known as Grading of cables

C.V.RAMAN GROUP OF INSTITUTIONS

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Grading of Cables

The unequal distribution of stresses is

undesirable because,

if dielectric is chosen according to maximumstress 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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Cables are generally laid in the ground or in ductsin the underground distribution system. For thisreason, there are little chances of faults in

underground cables. However, if a fault doesoccur 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 cables1) open circuit fault2) short circuit fault3)earth fault



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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 earthis measured by a megger and it will indicate zeroresistance in the circuit of the conductor that isnot broken.

However, if the conductor is broken, the megger

will indicate infinite resistance in its circuit When there is a break in the conductor of a cable,

it is called open circuit fault.



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When two conductors of a multi-core cable comein electrical contact with each other due toinsulation failure, it is called a short circuit fault.

Again, we can seek the help of a megger tocheck this fault.

For this purpose the two terminals of the meggerare connected to any two conductors.

If the megger gives zero reading, it indicatesshort circuit fault between these conductors.

The same steps is repeated for other conductorstaking two a time.



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EARTH FAULTS

When the conductor of a cable comes incontact with earth, it is called earth fault orground fault.

To identify this fault, one terminal of themegger is connected to the conductor andthe other terminal connected to earth.

If the megger indicates zero reading, it meansthe conductor is earthed. The sameprocedure is repeated for other conductors ofthe cable.



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Physical Limitations of

Underground LinesThe main argument against constructingunderground systems is usually financial. But costsare not the only limitation.The laws of physics limit how physically long a powerline can be.

These limits are relatively unimportant on overheadlines but will severely limit high voltage undergroundcable systems The higher the voltage the shorter the line length

must be. The limiting effects become very important at

transmission voltages, especially 100,000 Volts andabove. Limiting effects may also be important for

subtransmission voltages, 69,000 Volts and 35,000Volts.



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Physical Limitations: The Effect of

Capacitanceo Capacitance causes current to flow even when no

load is connected to the cable. This is called linecharging current.

o

Underground line capacitance for power cables is farhigher than overhead line capacitance.o Wires are closer to each othero Wires are closer to the earth (within a few inches).

o Underground lines have 20-75 times the linecharging current that an overhead line has

(depending on line voltage).o If a line is long enough the charging current could be

equal to the total amount of current the line cancarry. This will severely limit its ability to deliverpower.



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Summary of Costs: Overhead vs.

Underground

Transmission: Underground may be 4-20times Overhead.

Sub transmission: Underground may be 4-20

times Overhead Distribution: Underground may be 2-10 times

Overhead

New underground may be cheaper thanoverhead in special conditions and costs varygreatly from utility to utility and place toplace.



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THANKS

PREPARED & PRESENTED BY:

ROSHAN PRADHAN

regd no1221227062

sec-2 , group-c

sem-7th,year-4th

ELECTRICAL ENGINEERING

C.V.RAMAN COLLEGE OF ENGINEERING

roshan new.ppt

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