transmission lines

1

E. A. Frimpong, KNUST, Department of Electrical and Electronic Engineering, Ghana

2. TRANSMISSION LINES

2.1 Introduction

Overhead lines are used whenever possible on grounds of economy. An overhead line utilises air

to insulate the bare conductors for the majority of its length, apart from the very small sections

connected to insulators on the line supports. The absence of further insulation and the relative

ease of construction results in an inexpensive method of transporting electric power compared

with an underground cable, particularly at MV and higher voltages.

2.2 Overhead conductors

Although aluminium-alloy conductors are generally used at the smaller conductor sizes for LV

and MV distribution, the more common type of overhead line conductor in service is made from

aluminium wires, which conduct the current, wound around a core of steel strands which provide

the mechanical strength for the whole conductor. This is designated as aluminium-conductor

steel-reinforced (ACSR) conductor. Aluminium-alloy conductors use aluminium alloy (instead

of steel) as the core with aluminium conductors. Aluminium alloy conductors have better

corrosion resistance, lighter weight, and simpler splices. Copper conductors are also used in LV

systems, however, due to their high cost and the possible stealing of the conductors, they are

giving way to aluminium conductors.

The following categories of conductors are available:

(a) All aluminium conductor (AAC)

(b) All aluminium alloy conductor (AAAC)

(c) Aluminium conductor, steel reinforced (ACSR)

(d) Aluminium conductor, aluminium alloy reinforce (ACAR)

(e) Copper conductor

The proper design and choice of conductor material and size is of considerable importance. An

over-dimensioning of the overhead line system can have serious financial implications for the

investment, whilst under-dimensioning might not serve operational (with respect to voltage drop)

or future expansion requirements as the load on the system increases.

2.3 Properties of overhead conductors

An ideal conductor system should possess some properties. These include:

(a) Capability of carrying the specified load current and short-circuit current

(b) High electrical conductivity (low impedance) – to reduce drops and losses, and hence

improve voltage regulation

(c) High tensile strength – to withstand mechanical stresses or forces on it due to its situation;

these forces comprise:

(i) forces due to its own weight and weight of any othe conductors or equipment that

depend on it for their support

(ii) atmospheric forces, i.e., wind and ice load

(d) Corona-free at the rated voltage

(e) Minimum number of joints

(f) Minimum number of supporting insulators

(g) Economical or low cost

2


All the above requirements are not found in a single material. Therefore while selecting a

conductor material for a particular case, a compromise must be made between the desire for low

cost and the required electrical and mechanical properties.

The two most suitable and common materials for overhead conductor systems are copper and

aluminium. Steel has been used, but it has the obvious limitations of poor conductivity and high

susceptibility to corrosion.

(a) Copper It is an ideal material for overhead lines owing to its high electrical conductivity and greater

tensile strength. It is always used in the hand drawn form as stranded conductors. It has high

current density, i.e., the current-carrying capacity of copper per unit cross-sectional area is quite

large. This leads to two advantages:

(i) Smaller cross-sectional area of conductor required and hence lower cost

(ii) Smaller cross-sectional area offered for wind loading

The copper metal is quite homogeneous, durable and has a high scrap value. However, copper is

very expensive and not readily available.

There is no doubt that copper is most ideal for transmission and distribution of electric power.

However, due to its higher cost and non-availability, it is rarely used for these purposes. As such,

the trend these days is to use aluminium in place of copper.

(b) Aluminium

Aluminium is cheap and light as compared to copper. Being light, aluminium conductors are

subject to greater swings and hence larger cross-arms are required. It has much smaller

conductivity and tensile strength. The lower conductivity necessitates increased cross-sectional

area. Due to lower tensile strength and higher coefficient of linear expansion of, the sag in

aluminium is greater than in copper.

Considering the combined properties of cost, conductivity, tensile strength, weight, etc.,

aluminium is gaining more edge over copper. It is particularly economical to use aluminium for

heavy-current transmission, where the conductor size is large and its cost forms a major portion

of the total cost of complete installation.

2.4 Construction

There are four basic components of an overhead transmission line. These are

(a) Conductors – they carry power from the sending-end to the receiving-end.

(b) Ground (or Shield) wires

(c) Insulators

(d) Supports – wooden poles, steel poles, reinforced concrete poles, towers

2.4.1 The earth wire

Earth wires are bear wires supported at the very top of transmission line towers. The radius of the

earth wire is smaller than that of the phase conductors because the earthwire carries very little(or

no) current. It performs the following functions: (a) improve the capability of the phase

conductors by reducing the series impedance of the conductors, (b) shield the phase conductors

3


from direct lightning strokes and thus minimise faults due to lightning strikes on them (they

intercept the lightning strikes and conduct them to ground).

The protective angle of the earth wire is defined as the angle between the normal through the

earth wire and a slanting line connecting the earth wire and the phase conductor.

Earth Wire

Phase

Conductor

angle protective

Figure 1: Protective angle of earth wire

The protective angle should not be more than 300 for sufficiently good shielding of the line

conductor. To achieve effective shielding for towers of low height and for towers in areas

susceptible to high rate of thunderstorms, two ground wires are employed.

2.4.2 Supports

The required properties of overhead line supports are:

(a) High mechanical strength to withstand the weight of conductors and loading due to wind

and/or (ice)

(b) Light weight without loss in mechanical strength

(c) Cheap in cost and economical to maintain

(d) Easy accessibility of conductors for maintenance

(e) Longer lifespan

The supports must keep the conductors at a safe height from the ground and at adequate distance

from each other. They must be strong enough to withstand not only the dead weight of the

conductors, but also the loads due to ice and other things that might adhere to them and to wind

pressure. The line supports used for transmission and distribution are of various types. These

include wooden poles, steel poles, reinforced concrete poles and lattice steel towers. The choice

of supporting structure for a particular case depends on the life span, cross-sectional area of

conductors, line voltage, cost and local conditions.

(a) Wooden poles

Wooden poles are made of treated wood (e.g. teak) and are suitable for lines of moderate cross-

sectional area and of relatively shorter spans, about 40 – 50 m. They are cheap, easily available,

provide insulating properties and are therefore widely used for distribution in rural areas as an

economic proposition.

Wooden poles have the tendency to rot below the ground level, causing foundation failure. To

prevent this, the portion of the pole below ground level is impregnated with preservative

compound like creosote oil. They have comparatively lower lifespan (20-25 years), less

4


mechanical strength and require periodic inspection. Wooden poles are therefore not

recommended for voltages above 20kV.

(b) Steel poles

Steel poles are normally employed for distribution in cities and they support longer spans (50 –

80 m). They posses greater mechanical strength, and have longer lifespan. They can be

galvanized or painted in order to prolong its life. Three types of steel poles exist: rail poles,

tubular poles and rolled steel joints. Steel poles are mostly now employed for in street lighting.

(c) Reinforced concrete poles Reinforced concrete poles have greater mechanical strength, longer lifespan and also permits

longer spans (80 – 100 m) than steel and wooden poles. They require little maintenance but have

heavy weight.

(d) Steel towers (Pylons)

Steel towers are mainly employed in HV and EHV systems. They support the conductors

themselves and are, of course, the most visual part of a transmission line. Due to a growing effort

to reduce the visual impact of transmission lines and to make towers more economical, a great

variety of tower designs and materials have evolved. Towers have the greatest mechanical

strength, longest lifespan, can withstand severe climatic conditions and permit much longer

spans (100 – 300 m). For Extra high voltages, the spans can be larger (370 – 460m). Tower

footings are usually solidly grounded.

2.5 Spacing between conductors

Conductors must be sufficiently spaced to prevent arc-over under strong wind conditions. The

spacing between conductors increases with increasing spans and increasing line voltage. The

spacing can be obtained by empirical formula.

2.6 Sag in overhead lines

While erecting an overhead line, it is very important that conductors are under safe tension. If the

conductors are too much stretched between supports in a bid to save conductor material, the

stress in the conductor may reach unsafe value, and in certain cases, the conductor may break

due to excessive tension.

In order to permit safe tension in the conductors, they are not fully stretched but are allowed to

have a dip or sag. The difference in level between the points of supports and the lowest point on

the conductor is called sag.

S sag S

O

A B

Figure 2: Sag in an overhead line

5


2.6.1 Sag calculation when supports are at equal levels.

S

O

A B

l

l2

1 l2

1

T

y

x

wx

x2

1

Figure 3: Sag calculation for supports at the same level

It must be recalled that tension is governed by conductor weight, wind effects, ice loading (not

applicable in Ghana) and temperature variations. A standard practice is to keep conductor tension

less than 50% of its ultimate tensile strength, i.e., a minimum factor of safety in respect of

conductor tension is 2.

Let l length of span

cw weight per unit length of conductor

T tension in the conductor

O reference point for measuring the coordinates of different points on the conductor.

Let us assume that the curvature is small, that is, the length of curved surface xOP , the

horizontal projection. Then the two forces acting on the portion OP of the conductor are:

(a) The weight xWc of the conductor acting at a distance2

x from O.

(b) The tensionT acting at O.

Mathematically, we have

T

wyor

xxwyT

xc

c

2

22

Eq. 3.1

For the minimum sag, the coordinates are: Syl

x ,2

. Substituting these values into equation

(3.1), we obtain:

T

wSSag lc

8

2

Eq. 3.2

The following points about sag must are worth noting:

6


(a) Conductor sag and tension are important considerations in the mechanical design of

overhead lines

(b) Sag should be kept minimum in order to reduce conductor material required, as well as

avoid extra pole height for sufficient conductor clearance above ground level.

(c) Sag and stresses vary with temperature on account of thermal expansion and contraction of

the line conductors.

(d) When a conductor is suspended between two supports at the same level, the sag-span curve

approaches that of a parabola, if the sag is very small compared with the span.

(e) Tension in the conductor material must be low to avoid mechanical failure.

(f) The tension at any point on the conductor acts tangentially, and so the tension at the

minimum point of sag is horizontal.

(g) The horizontal component of tension is constant throughout the length of the wire

(h) The tension at supports is approximately equal to the horizontal tension acting at any point

on the wire.

2.6.2 Sag calculation when supports are at different levels

1S

A

B

h

2S

O1x 2x

l

Figure 4: Sag calculation for supports at different levels

From equation (3.1), the sag at support A is given as

T

XWS c

2

2

1

1 Eq. 3.3

Similarly at support B, the sag is given as

T

XWS c

2

2

2

2 Eq. 3.4

But lxx 21 , Hence,

xx

xxxx

xxSS

T

lw

T

w

T

w

c

c

c

12

1212

21

2212

2

2

2

Eq 3.5

If hSS 12 , then

7


lw

Th

cxx

212

Eqn 3.6

Combining equations (3.5) and (3.6), we obtain

lw

Thl

cx

22

lw

Thl

cx

21

Once the values of 1x and 2x are determined, the sags 1S and 2S can be easily found.

2.6.3 Effect of wind and ice loading on sag calculation

The expressions derived for sag earlier are only true if the conductor were acted upon by its

weight only. In practice, however, a conductor is always subjected to wind pressure and, in the

temperate regions, to ice loading or coating.

t

d

Ice skating

Wind

ww

tw

wwi

Figure 5: Effect of ice coating and wind

Ice coating acts vertically downwards while wind loading acts horizontally, i.e., at right angles to

the projected surface of the conductor

If we denote iw as the weight of ice per unit length, cw the weight of conductor per unit length

and wt the total weight of conductor per unit length, then

wwww wt ic22

)(

But lengthunitpericeofVolumeiceofDensitywi

1)2(4

22 dtdiceofDensitywi

dtticeofDensity

Also lengthunitperareaprojectedareaunitperpressureWindww

)2(

12

tdpressurewind

tdpressurewind

The conductor sets itself in a plane at an angle to the vertical, where

8


w

w

ic

w

w tan

The slant sag in the conductor is given by the relation

T

lwS

t

slant8

2

where slantS represents the “slant sag” in a direction making an angle to the vertical. If no

specific mention is made of the problem, then the slant sag is calculated by using the above

formula.

cos sagslantsagVertical

Example 2.1

A 132kV transmission line has the following data:

Weight of conductor kmkg680

Ultimate strength kg3100

Span m260

Safety factor 2

Calculate the height above ground at which the conductor should be supported. The ground

clearance required is 10m.

Solution

mkgwc 68.01000

680

kgT 15502

3100

mT

lwSag c 7.3

15508

26068.0

8

22

Conductor should therefore be supported at a height of m7.137.310

Example 2.2

A transmission line has a span of 150m between level supports. The conductor has a cross

sectional area of 22cm , the tension in the conductor is 2000kg. if the density of the conductor

material is 399.9 cmg and wind pressure is mkg5.1 length, calculate the sag. What is the

vertical sag.

Solution

Weight of conductor per meter kgcmcmg 98.11000

100299.9 23

Total weight mkgwww wct 48.25.198.1 2222

48.320008

15048.2

8

22

T

lwSag t

For vertical sag, '143798.1

5.1tan

98.1

5.1tan 01

c

w

w

w

9


msagVertical 77.2'1437cos48.3 0

Example 2.3

The effective diameter of a line is 1.96 cm and it weighs 90 kg per 100 m length. If the line is

subjected to a loading due to ice of radial thickness 1.25 cm and density 920 kg/m3, as well as a

horizontal wind pressure of 30 kg/m2, and the maximum stress in the line is not to exceed ¼ of

the ultimate strength of 4000 kg/cm2, then calculate (a) additional loading due to the ice and

wind pressure, (b) total weight per metre run of the ice, (c) maximum sag in still air, and (d)

maximum vertical sag under additional loading due to ice and wind.

Solution

(a) Additional loading due to ice and wind pressure

mkgrdrw iiii / 16.110)25.196.1(1025.1920)( 22

mkgrdw iww / 34.110)25.1(296.130)2( 2

(b) Total weight per metre run

mkgwwww wcir / 46.234.1)9.016.1()( 2222

(c) Maximum sag

kgcmkgareastressimumT 58.30174

96.1/ 4000

4

1 max

22

cmm

T

lwS c 373 373.0

58.30178

1009.0

8

22

0

(d) Maximum sag under additional ice and wind pressure loading

The sag under additional loadings is

m

T

lwS r

r 02.158.30178

10046.2

8

22

The vertical sag is

cmmw

wwSSS

r

ic

rrvertical 854 854.046.2

16.19.002.1cos

2.7 Current rating of conductors

The current capacity of overhead conductors is decided by thermal, i.e. the temperature rise,

considerations. Because of electrical limitations, long lines are not usually operated close to their

thermal rating especially with bundled conductors. The conductor operating temperature is

limited (70oC is often used) by mechanical aspects such as allowable mid-span sag, creep in the

conductor, and long-term mechanical effects. Perhaps the most limiting aspect on lines is the

temperature of joints and clamps. Heat is dissipated in overhead lines by radiation and

convection.

2.8 Insulators

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Overhead line conductors should be supported on the poles and towers in such a way that current

from conductors do not flow to earth through the supports. The insulators provide the necessary

insulation between line conductors and supports. The desirable properties of insulators are:

(a) High mechanical strength to withstand the line load.

(b) High electrical resistance to surface leakage current. To increase the leakage path and

hence the leakage path resistance, the insulators are moulded in the shape of a skirt.

(c) Sufficient thickness to prevent puncture under the high voltage stresses they have to

withstand.

(d) High dielectric strength or relative permittivity

(e) Non-porous, free from impurities and cracks, so as not to lower the permittivity

The most commonly used material for insulation of overhead lines is porcelain, but glass and

other synthetic materials are also used. The most common types of insulators are (i) pin type (ii)

suspension type (iii) strain insulators (iv) Shackle insulators

2.8.1 Pin-type insulators

Groove for

conductor

Galvanised

Steel Pin Figure 6: Pin-type insulators

It has several porcelain skirts and the conductor is fixed at the top in a groove. A steel pin screws

into the insulator so that it can be bolted to the support. The skirts are fixed together by portland

cement. This type is used for lines up to about 50kV. They may be constructed in one piece for

voltages to about 25kV and in two or three pieces from 25kV to 50kV.

2.8.2 Suspension-type

String of

Insulators

Line

Conductor Figure 7: Suspension-type insulators

Suspension type insulators are known as disc or string insulators. They are employed for higher

transmission voltages (greater than 33kV). They are not generally used on distribution circuits

except at turning points and dead ends where pin-type may not provide sufficient strength.

Suspension insulators consist of a number of discs made of glass or porcelain strung together by

ball and socket metallic parts. Each disc in the string is usually designed for 11 kV. A number of

11


them are connected in series to form a chain and the line conductor is carried by the bottommost

insulator. The main advantages of the suspension type insulators are:

(a) Suspension type insulators are cheaper than pin type insulators for voltages higher than 33

kV.

(b) The conductors run below the earthed cross-arm of the tower, thereby providing protection

against lightning.

(c) Ease of selecting the number of discs for a particular voltage.

(d) If any disc is damaged, the whole string does not become useless because the damaged disc

can be easily replaced.

(e) In the case of increased demand on the transmission line, it is found more satisfactory to

supply the greater demand by raising the line voltage than to provide another set of

conductors. The additional insulation required for the raised voltage can be easily obtained

in the suspension arrangement by adding the desired number of discs.

2.8.3 Strain-type insulators Suspension type insulators, when used in horizontal position, are known as strain insulators.

They are used to handle the tension off conductors at sharp curves/corners or at the end of

transmission lines or at anchor towers where lines are dead ended at road or river crossings.

2.8.4 Shackle insulators

They are used for low voltage distribution lines. They can be used either in the horizontal or in

the vertical position. They can be directly fixed to the pole with a bolt or to the cross arm. For

voltages up to 11kV, they are generally employed at turning points and dead ends.

Figure 8: Shackle insulator

2.8.5 Potential distribution over suspension insulator string

A string of suspension insulators consists of a number of units connected in series through

metallic links, the number of units depending on the transmission voltage. Each insulator units

constitutes a capacitor. There exists therefore mutual or self- capacitance between different units.

In addition, there exists capacitance to ground (tower) because of the nearness of the tower,

cross-arm and line.

Due to this shunt capacitance to ground, the charging current is not the same through all the

units. Consequently, the total system voltage is not equally distributed over the different units of

the string. See the Fig below.

12


1V

2V

3V

V

Conductor

C

C

C

A

B

D

1I

2I

3I

Conductor

1i

2i

3i

kC

kC

kC

Figure 9: Suspension insulator string and voltage distribution

Concerning the suspension insulator string, the following points must be noted:

(a) The voltage impressed on a string of suspension insulators does not distribute itself equally

across the individual insulator units.

(b) The inequality of the voltage distribution between individual units becomes more

pronounced as the number of insulator units increases, and it is also dependent on the ratio

(capacitance of insulator/capacitance of earth).

(c) The unit closest to the line conductor carries the maximum percentage of the line voltage,

the figure progressively decreasing as the unit nearest to the tower is approached.

(d) The unit nearest to the conductor is under maximum stress and is likely to be punctured

first.

2.8.6 Calculation of potential distribution along insulator units

Suppose that the shunt capacitance to ground is a fraction k of the self-capacitance C of each

insulator unit. That is;

C = self capacitance of insulator unit

kC = shunt capacitance to ground (tower)

It is seen that VCC/1

VI 1

11

Similarly VCkkC/1

Vi 1

11

Applying Kirchoff’s current law (KCL) at node A,

)1k(VCiII 1112

)1k(VV

CkVCVCV

12

112

The shunt current i2 is produced by the voltage combination of ( 21 VV ).

Appyling KCL at node B,

13


iII 223

)1k3k(V

)1k()1k(VkV

)1k(VkVV

Ck)VV(CVCV

21

11

213

2123

Similarly, the voltages on the nth unit can be determined.

2.8.7 String efficiency of string insulators

Owing to the proximity of the supporting structure which is not at earth potential, the distribution

of voltage along the insulator string is not uniform. The disc nearest to the conductor is most

highly stressed.

If the string has n units, the total voltage across the string is given as:

V..........VVVVV n4321

The string efficiency is then given by

line the tounit theacross voltage

string theacross voltagetotal efficiency String

closestn

The string efficiency is an important consideration, since it decides the potential distribution

along the string. The greater the string efficiency, the more uniform is the voltage distribution.

Thus 100% efficiency is an ideal case for which the voltage across each unit will be exactly the

same. Although it is impossible to achieve 100% string efficiency, it can be improved.

2.8.8 Improving the string efficiency

The inequality of voltage distribution along the string increases with the number of units in the

string. Therefore shorter strings have more efficiency. The greater the value of k, (i.e., the ratio

of capacitance to earth/capacitance per unit), the more non-uniform is the potential distribution

across the string, and hence the lesser is the efficiency.

If the insulation of the most stressed insulator unit is punctured or flash over takes place, the

breakdown of the other units will take place in succession. There is therefore the need to equalize

the voltage distribution across the various units, i.e., to improve the string efficiency.

The string efficiency can be improved by:

Making the ratio of capacitance to earth/capacitance per unit as small as possible, through

longer cross-arms.

Use of graded insulators

Providing a guard ring to surround the lowermost unit, which is connected to the metal

work at the bottom. This ring increases the capacitance between the metal work and the

line, and helps in equalizing the voltage distribution along the different units.

(a) Longer cross-arms

The value of the string efficiency depends on the value of k, i.e., the ratio of shunt capacitance to

self-capacitance. The lesser the value of k, the greater the string efficiency, and the more uniform

is the voltage distribution. The value of k can be decreased by reducing the shunt capacitance.

14


The shunt capacitance is reduced by increasing the distance of the conductor from the tower,

i.e., the use of longer cross-arms. The extent of the reduction is, however, limited by cost and

the mechanical strength. In practice, k = 0.1 is the limit achievable by this method.

Conductor Figure 10: Reduction of shunt capacitance

(f) Use of graded insulators

The insulators are capacitance graded, that is, they are assembled in the string in such a manner

that the top unit has the minimum capacitance, increasing progressively towards the bottom unit

that is nearest to the conductor. Since voltage is inversely proportional to capacitance, this

method tends to equalize the potential distribution across the units in the string. The main

problem is that it requires different-sized insulators.

(g) Use of guard Ring Guard rings are metal rings electrically connected to the conductor and surrounding the bottom

insulator. See Figure below.

A

B

1I

2I

3I

1i

2i

Conductor

1

1

i

1

2

i

Guard

Ring

Figure 11: Use of guard ring for uniform voltage distribution

The guard ring increases the capacitance between the metal fittings and the line conductor which

was neglected in the earlier model. The guard is contoured in such a way that the shunt

capacitance currents 1i , 2i , 3i etc., are equal to the metal fitting line capacitance currents. This

results in the same charging current flowing through each unit of the string. Consequently, there

will be uniform potential distribution across the units.

15


Example 3.4

Each line of a 3-phase 11 kV system is suspended by a string of 3 identical insulators. The self-

capacitance of each disc is found to be 10 times the shunt capacitance to ground. Find the voltage

distribution across each insulator and the string efficiency.

Solution

1V

2V

3V

V

Conductor

C

C

C

A

B

D

1I

2I

3I

Conductor

1i

2i

3i

kC

kC

kC

The phase voltage is given as kV 35.6732.1

11

3

VV L

ph

)1(12 kVV and )13( 2

13 kkVV

Where 1.010

1

self

shunt

C

Ck

But phVVVV 321

Hence,

kVV

VVV

kkVkVV

86.1

35.61)1.0(31.0)11.0(

35.6)13()1(

1

2

111

2

111

Hence, kVVV 05.286.11.11.1 12 kVVV 44.286.131.131.1 13

String efficiency,

%7.86867.044.23

35.6

3 3

V

V

Vn

V ph

n

ph

string

2.8.9 Contamination of insulator surfaces

Dust, acids, salts and other pollutants settle on the insulators. When the pollution layer becomes

moist due to rain, fog or dew, it becomes conductive and a surface leakage current of much

higher magnitude than the dry value flows. Insulator pollution may lead to short circuits during

storms or under momentary overvoltage conditions.

16


To prevent flash over due to pollution, the leakage length of the insulator is increased. Greasing

of the entire insulator surface prevents the formation of a continuous film. However, a fresh

application of grease is required periodically, say 1 to 2 years. The washing of surfaces with

water from hoses when the equipment is live is also effective.

Insulators are required to withstand both mechanical and electrical stresses. The latter type is

primarily due to excessive line voltage and may cause the breakdown of the insulator. Electrical

breakdown of the insulator can occur either by flashover or puncture.

In flashover, an arc occurs between the line conductor and insulator pin (earth), and the

discharge jumps across the air gaps, following the shortest distance. The insulator will, however,

continue to act in its proper capacity unless extreme heat produced by the arc destroys the

insulator.

In the case of puncture, the discharge occurs from conductor to pin through the body of the

insulator. Puncture breakdown permanently destroys the insulator due to excessive heat. The

ratio of puncture strength to flashover voltage is known as safety factor.

2.9 Corona discharge

When an alternating potential difference is applied across two conductors whose spacing is large

compared to their diameters, there is no apparent change in the condition of the atmospheric air

surrounding the wires, if the applied voltage is low.

However, when the applied voltage exceeds a certain value, called the critical disruptive voltage,

the conductors are surrounded by a faint luminous glow of bluish colour called corona. The

phenomenon of corona is always accompanied by (i) a hissing sound production (ii) production

of ozone which is easily detected because of its characteristic odour and (iii) radio interference.

The higher the voltage is raised above the critical disruptive value, the large and higher the

luminous envelope becomes, and the greater the hissing sound, the power loss and the radio

noise.

If the applied voltage is further increased to the breakdown value, the hissing and glow increase

so much so that a flashover or spark will occur between the conductors due to the breakdown of

air insulation. If the conductors are polished and smooth, the corona glow will be uniform

throughout the entire length; otherwise the rough points will appear brighter.

If the transmission voltage is DC instead of alternating, there will exist a difference in the

appearance of the two wires. The positive wire will have a uniform glow about it, whilst the

negative conductor will have spotty glow.

2.9.1 Disadvantages of corona

(a) Corona is accompanied by loss of energy (dissipation of power), although it is not so

important except under abnormal weather conditions like storms, etc. This loss of energy

affects the transmission efficiency of the line.

(b) Ozone produced as a result of corona may react chemically with the conductor and cause

corrosion of the conductor.

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(c) The current drawn by the line due to corona losses is non-sinusoidal, and hence non-

sinusoidal voltage drop occurs in the line. This may cause interference with neighbouring

circuits due to electromagnetic and electrostatic induction.

(d) Corona current tends to introduce a large third harmonic component.

Corona effects are intense for voltages of 35 kV and higher. Corona discharges round busbars are

extremely undesirable, because the intense ionization of the air reduces its dielectric strength

and makes it easier for flashover to occur in insulators and between phases, particularly when

the surfaces concerned are dirty or soiled with other deposits/impurities.

Notwithstanding these unwanted effects of corona, it produces some desirable effects

(a) Due to corona formation, the air surrounding the conductor becomes conducting and hence

the virtual diameter of the conductor is increased. The increased diameter reduces the

electrostatic stresses between conductors.

(b) Corona reduces the effects of transients produced by surges.

To minimise corona in lines using one conductor per phase, an expanded (larger diameter)

conductor has been developed. In one method, non-metallic strands of fibrous material are

inserted between the steel core and aluminium conductor wires. Another incorporates air pockets

formed by wrapping aluminium strands helically around the steel core. The usual way to reduce

corona with its attendant problems of audible and radio noise is to use more than one conductor

per phase forming a bundle. The effect of having two or more sub- conductors per phase is to

reduce the maximum surface electric field and hence the magnitude of corona discharge. A

further effect of bundled conductors is reduced inductive reactance. The sub-conductors are

spaced apart by steel spacers at frequent intervals along the line. Three or four conductors are

frequently used in the 400 – 765kV range.

2.10 Vibration

If a coating of sleet (partly melted falling snow) is deposited on a line during windy conditions,

the line may begin to oscillate (frequency of oscillation is about 1Hz). Under certain conditions

(for example when there is irregular coating of sleet), the oscillations may become so large

(amplitude of about 6meters) that the line is seen to actually “gallop” or “dance”. Galloping lines

can produce short circuits between phases or snap the conductors. High frequency oscillations (5

to 100Hz of about 25mm amplitude) are also produced in lines due to the formation of eddies.

The conductor vibrates in a number of loops, depending on it’s length, mass per unit length and

tension. This vibration can cause the conductor to suffer fatigue and eventually break at the

clamps of supports.

To eliminate the problem of vibration, lines are sometimes loaded with special mechanical

weights to dampen the oscillations or to prevent them from building up.

Figure 12: A vibration damper

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Another type of damper different from the one shown in the figure above consist of a box

containing a weight resting on a spring. The fatigue may also be reduced by reinforcing the

conductor for a few meters on either side of the clamp by binding metal rods or same length of

the same conductor to the main conductor outside.