underground cables

1

UNIT- V

Underground Cables

Introduction

2

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.

Comparison of OH lines & UG Cables

3

Public safety

Initial cost

Maintenance cost

Frequency of Faults or Failures

Frequency of accidents

Voltage drop

Charging current

Appearance

Fault location and repairs

Jointing & Tapping

Damage due to Lightning & Storm

Interference to communication circuits

Introduction

4

Advantages & Disadvantages

5

Advantages

Better general appearance

Less liable to damage through storms or lightning

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.

Construction of Cables

6


7

Core or ConductorA cable may have one or more than one coredepending upon the type of service for which it isintended. The conductor could be of aluminum orcopper and is stranded in order to provide flexibilityto the cable.

InsulationThe core is provided with suitable thickness ofinsulation, depending upon the voltage to bewithstood by the cable.The commonly used material for insulation areimpregnated paper, varnished cambric or rubbermineral compound.


8

Metallic Sheath

A metallic sheath of lead or aluminum is provided over theinsulation to protect the cable from moisture, gases or othersdamaging 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.


9

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.

CABLE STRUCTURE

10

Properties of Insulating Material

11

Properties of Insulating Material

12

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.

Insulating Materials for Cables

13

• 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


14

• Vulcanized Indian Rubber (VIR)

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.


15

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.


16

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.

XLPE cable

17

CLSSIFICATION OF CABLES

18

According to no. of conductors

Single, Two, Three or Four core cables

According to the operating voltage

Low tension (L.T) ----- up to 1 kV

High tension (H.T) ----- up to 11 kV

Super tension (S.T) ---- from 11 kV to 33 kV

Extra high tension (E.H.T) cables --- from 33 kV to 66

kV

Extra super voltage cables ------beyond 132 kV

19

According to the method of insulation & sheathing

provided

Belted type

H-type

SL type

HSL type

According to the method of improving the dielectric

stress

Solid type

Oil filled type

External oil pressure type

Gas pressure type

CLSSIFICATION OF CABLES

Low Tension Cable

20

Extra High Tension Cable

21

3- Core Cables

22

Belted 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 beused 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

3-core belted Cable

23

3- Core Cables

24

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

3- Core Cables

25

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 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 airpockets or voids in the dielectric is eliminated.

The metallic screen increase the heat dissipation power of thecable.

3- Core Cables (H-Type)

26

3- Core Cables

27

S.L -Type: (Separate Lead)

Each core insulation is covered by its own lead sheath.

It has two main advantages, firstly the separate sheath minimizethe possibility of core-to-core breakdown. Secondly the,bending of cables become easy due to the elimination of overall sheath.

The disadvantage is that the lead sheaths of S.L is much thinneras compared to H-Type cables, therefore for greater care isrequired in manufacturing.

3- Core Cables (S.L. Type)

28

3- Core Cables

29

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.

3- Core Cables (Gas pressure)

30

3- Core Cables

31

Oil Filled Cables 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

a. Self-contained circular type

b. Self-contained flat type

c. PipeType cables

3- Core Cables (Oil filled)

32

Advantages of Oil Filled Cables

33

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

Gas Pressure Cables

34

In these cables an inert gas like nitrogen is used to exert pressure

on paper dielectric to prevent void 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 known as pipe line compression cable.

The nitrogen gas is filled in vessel at nominal pressure of 1.38 *

10 exp 6 N/ square meter with a maximum pressure of 1.725 *

10 exp 6 N/ square meter.

Gas Pressure Cables

35

Gas Insulated Cables (GIC)

36

In GIC cables high pressure sulphur hexaflouride (SF6), fills

the small spaces 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.

Gas Insulated Cables (GIC)

37

Advantages of GIC

38

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.

Laying of Underground Cables

39

The reliability of underground cable network depends to a

considerable extent upon proper laying.

There are three main methods of Laying underground

cables

a. Direct Laying

b. Draw in system

c. Solid system

Direct Laying

40

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

41

Disadvantages of Direct Laying

42

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

43

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.

Advantages of Draw in System

44

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

Solid System

45

In this system the cable is laid in open pipes or troughs 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 weather conditions

It is very much expensive system

Solid System

46

Grading of Cables

47

• 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

Grading of Cables

48

• 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

49

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 fault

because 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

50

When there is a break in the conductor of a cable, it iscalled open circuit fault.

The open circuit fault can be checked by megger. Forthis purpose, the three conductors of the 3-core cable atthe far end are shorted and earthed.

The resistance between each conductor and earth ismeasured by a megger and it will indicate zeroresistance in the circuit of the conductor that is notbroken.

However, if the conductor is broken, the megger willindicate infinite resistance in its circuit

51

When two conductors of a multi-core cable come in

electrical contact with each 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 two conductors.

If the megger gives zero reading, it indicates short circuit

fault between these conductors.

The same steps is repeated for other conductors taking two a

time.

EARTH FAULTS

52

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.

Physical Limitations of

Underground Lines

53

The main argument against constructing undergroundsystems is usually financial. But costs are not the onlylimitation.The laws of physics limit how physically long a powerline can be.These limits are relatively unimportant on overhead linesbut will severely limit high voltage underground cablesystems The higher the voltage the shorter the line length must be. The limiting effects become very important at transmission

voltages, especially 100,000Volts and above. Limiting effects may also be important for subtransmission

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

Physical Limitations: The Effect

of Capacitance

54

o Capacitance causes current to flow even when no load isconnected to the cable. This is called “line chargingcurrent”.

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 line chargingcurrent that an overhead line has (depending on linevoltage).

o If a line is long enough the charging current could beequal to the total amount of current the line can carry.This will severely limit its ability to deliver power.

Summary of Costs: Overhead vs.

Underground

55

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.

Engineering

underground cables