CHAPTER-1

TRANSMISSION LINE

1.1 INTRODUCTION

Electrical power is the basic need for the economic development of any country. The

energy consumption is the main index for the overall development and growth of a

country. The process of modernization, increase in productivity in industry and

agriculture and the improvement in the standard of living of the people basically depend

on the adequate supply of electrical energy.

The electrical energy is generated by hydroelectric power plants, thermal power plants

and nuclear power plants. The electrical power is transmitted from these power plants to

the consumer’s premises by using transmission and distribution systems. The power from

the generating station is transmitted at high voltage (such as 132, 220, 440 kV) over long

distances to the major load centres. The line should have sufficient current carrying

capacity so as to transmit the required power over a given distance without excessive

voltage drop and overheating. The line losses should be small and insulation length

should be adequate to cope with the system voltage.

The transmission system of an area is known as ‘GRID’. The different grids are

interconnected through the ‘TIE’ lines to form a ‘regional grid’ and the different regional

grids are further interconnected to form a ‘national grid’. Each grid operates

independently. Power can be transmitted from one grid to another, over the tie lines under

the condition of sudden loss of generation or increase in load.

A single phase AC circuit requires 2 conductors. A 2-phase AC circuit using same size

conductor as a 1-phase circuit can carry 3 times the power which can be carried by a

single phase circuit and uses 3 conductors of 3-phase and 1-conductor of neutral. Thus a

3-phase circuit is more economical then a 1-phase circuit in terms of initial cost as well as

the losses. All transmission and distribution systems are, therefore, 3-phase systems. In

fact, a balanced 3-phase circuit does not require the neutral conductor as the instantaneous

sum of the 3 line currents is zero.

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Therefore, the transmission lines and feeders are 3-phase, 3-wire circuit. The distributors

are 3-phase, 4-wire circuit because a neutral wire is necessary to supply the 1-phase load

for domestic and commercial consumers. The standard frequency in India and many other

countries is 50 Hz.

The overhead line conductors are bare and not covered with any insulating covering

coating. The line conductors are, therefore, secured to the supportive structures by means

of insulating fixtures, called the insulators, in order that there is no current leakage to the

earth through the supports. The material most commonly used for overhead line is

‘Porcelain’. But toughened glass, steatite and special composition materials are used to

limited extent. Insulators are required to withstand both electrical and mechanical

stresses.

In the present work, we have designed a 3-phase transmission system to transmit a given

power through a given distance. Subjected to the constraints such as efficiency and

regulation for a given power factor of the load. We have also attempted mechanical

design of a transmission line. The mechanical design comprises of selection and number

of insulators, proper sag and minimum distance of the line from the ground and based on

this, we have selected a suitable tower.

1.2 HISTORY OF TRANSMISSION LINE

Before we dig deep into the principles of Transmission Line Losses, let us first review a

brief history of the power transmission line particularly with Overhead Transmission

Line.

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Fig no. 1.1: View of a transmission line.

The first transmission of electrical impulses over an extended distance was demonstrated

on July 14, 1729 by the physicist Stephen Gray, in order to show that one can transfer

electricity by that method. The demonstration used damp hemp cords suspended by silk

threads (the low resistance of metallic conductors not being appreciated at the time).

However the first practical use of overhead lines was in the context of telegraphy. By

1837 experimental commercial telegraph systems ran as far as 13 miles (20 km). Electric

power transmission was accomplished in 1882 with the first high voltage transmission

between Munich and Miesbach. 1891 saw the construction of the first three-phase

alternating current overhead line on the occasion of the International Electricity

Exhibition in Frankfurt, between Lauffen and Frankfurt.

In 1912 the first 110 kV-overhead power line entered service followed by the first 220

kV-overhead power line in 1923. In the 1920s RWE AG built the first overhead line for

this voltage and in 1926 built a Rhine crossing with the pylons of Voerde, two masts 138

meters high.

In Germany in 1957 the first 380 kV overhead power line was commissioned (between

the transformer station and Rommerskirchen). In the same year the overhead line

traversing of the Strait of Messina went into service in Italy, whose pylons served the

Elbe crossing 1. This was used as the model for the building of the Elbe crossing 2 in the

second half of the 1970s which saw the construction of the highest overhead line pylons

of the world. Starting from 1967 in Russia, and also in the USA and Canada, overhead

lines for voltage of 765 kV were built. In 1982 overhead power lines were built in Russia

between Elektrostal and the power station at Ekibastusz, this was a three-phase

alternating current line at 1150 kV (Power line Ekibastuz-Kokshetau). In 1999, in Japan

the first powerline designed for 1000 kV with 2 circuits were built, the Kita-Iwaki

Powerline. In 2003 the building of the highest overhead line commenced in China, the

Yangtze River Crossing.

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CHAPTER-2

COMPONENTS OF A TRANSMISSION LINE

2.1 INTRODUCTION

The transmission lines are like the arteries of the power system. Transmission lines act as

medium for carrying bulk energy from one substation to other. The electric energy

transmission is carried out at High and Extra High Voltages (EHV). Voltage above 220

kV is usually referred as Extra High Voltage. The transmission lines can be constructed

over head or underground. The overhead lines are bare conductors with proper clearances

from earthed structures and between the phase conductors.

2.2 TRANSMISSION SYSTEM REQUIREMENTS

Listed below are the typical points to be considered before starting or even operating an

Electrical Power System. These factors can be best categorized into three main points;

Electrical Design, Mechanical Design & Structural Design.

Electrical Design of AC system involves;

power flow requirements

system stability and dynamic performance

selection of voltage level

voltage and reactive power flow control

conductor selection

losses

corona-related performance(radio, audible, and television noise)

electromagnetic field effects

insulation and over voltage design

switching arrangements

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circuit-breaker duties

protective relaying.

Mechanical Design includes

Sag and tension calculations

conductor composition

conductor spacing (minimum spacing to be determined under electrical design)

types of insulators

selection of conductor hardware

Structural Design

selection of the type of structures to be used

mechanical loading calculations

foundations

guys and anchors.

Miscellaneous features

line location

acquisition of right-of-way

profiling

locating structures

inductive coordination (considers line location and electrical calculations)

means of communication

2.3 HARDWARE COMPONENTS OF TRANSMISSION LINE

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The following are the most common overhead transmission line components:

Structures for Support (Poles & Towers)

Wires and Cables (phase conductors & OHGW)

Insulators (ceramics & polymer)

Connectors

Guying for support

Line Arresters

Others (vibration damper, corona ring, spacers, etc.

2.4 CONDUCTORS IN TRANSMISSION LINE

In the past, electric power was transmitted through the use mostly of copper conductors.

Copper is rank among the most ideal metals for transmitting electricity due to its low

resistivity also, of which it is second to silver. However, in the modern days, aluminum

replaced copper as a main material for transmitting electricity simply because of the much

lower cost and lighter weight of an aluminum conductor in contrast to a copper conductor

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Fig no. 2.1: Picture showing most common components of a transmission line.

with the same resistance. Another advantage of an aluminum is when compared to a

copper with the same resistance, aluminum tends to have a larger diameter. It is an

advantage because with a conductor with a relatively larger diameter the lines of electric

flux originating on the conductor will be farther apart at the conductor surface for the

same voltage.

Electrical conductor

In physics and electrical engineering, a conductor is a material which contains movable

electric charges. In metallic conductors such as copper or aluminum, the movable charged

particles are electrons. Positive charges may also be mobile in the form of atoms bound in

a crystal lattice which are missing electrons (known as holes), or in the form of mobile

ions, such as in the electrolyte of a battery, or as mobile protons in proton conductors

employed in fuel cells. In general use, the term "conductor" is interchangeable with

"wire."

Physics

All conductors contain electric charges, which will move when an electric potential

difference (measured in volts) is applied across separate points on the material. This flow

of charge (measured in amperes) is what is meant by electric current. In most materials,

the direct current is proportional to the voltage (as determined by Ohm's law), provided

the temperature remains constant and the material remains in the same shape and state.

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Fig no. 2.2 View of overhead conductors carry electric power.

2.4.1 CONDUCTOR MATERIALS

Copper has a high conductivity. Silver is more conductive, but due to cost it is not

practical in most cases. Because of its ease of connection by soldering or clamping,

copper is still the most common choice for most light-gauge wires. Aluminum has been

used as a conductor in housing applications for cost reasons. It is actually more

conductive than copper when compared by unit weight, but it has technical problems that

have led to problems when used for household and similar wiring, sometimes having led

to structural fires:

A tendency to form an electrically resistive surface oxide within connections,

leading to heat cycling of the connection (unless protected by a well-maintained

protective paste);

A tendency to "creep" during thermal cycling, causing connections to become

loose due to a low mechanical yield point of the aluminum; and

A coefficient of thermal expansion sufficiently different from the materials used

for connections, accelerating the creep problem and addressed by using only

plugs, switches, and splices rated specifically for aluminum.

These problems do not affect other uses, and aluminum is commonly used for the low

voltage "drop" between a power pole and the household meter. It is also the most

common metal used in high-voltage transmission lines, in combination with steel as

structural reinforcement.

Listed below are some of the known types of aluminium conductors that are used by

many transmission and distribution utility worldwide;

AAC All-Aluminium Conductors

AAAC All-Aluminium-Alloy Conductors

ACSR Aluminium Conductor, Steel Reinforced

ACAR Aluminium Conductor, Alloy Reinforced

Due to the low tensile strength of aluminium, experts created a way to fill this void. They

were able to create a higher tensile strength conductor by incorporating aluminium with

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other types of metal. ACSR which consists of a central core of steel strands surrounded

by layers of aluminium strands is now the type of configuration that are popularly used as

conductors for transmission lines.

The most common conductor materials are hard drawn copper and aluminium. Their

properties are given in table 2.1.

Table 2.1: Properties of Copper and Aluminium conductors

Copper Aluminium

Electrical conductivity (silver = 1.0) 0.975 0.585

Resistivity (μ Ω-cm) 1.777 2.826

Specific gravity 8.89 2.70

Tensile strength( ) 3.84 to 180 to 234

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Fig. no. 2.3 Aluminium with Steel

Fig. no. 2.4 Different kind of ACSR cables according to composition

430

Coefficient of linear expansion per 17 23

Temperature coefficient of resistance

at 20

0.00393 per 0.004

Ratio of conductivities for equal area 1 0.6

Ratio of diameters for equal

resistance

1 1.29

Ratio of weights for equal resistance 2 1

2.4.2 TYPES OF CONDUCTOR

1. Stranded Hard Drawn Copper. Hard drawn copper has the advantages of very

high conductivity (i.e., very low resistivity), good tensile strength and weather

resisting properties. Many years back it was widely used for construction of

overhead lines. Due to non-availability and high cost involvement, it is generally

not use in India. In other countries, too, it is very rarely used.

2. Aluminium. Aluminium has the advantages of much lower cost and lesser weight

as compared to copper. The fact that an aluminium conductor has a larger

diameter than a copper conductor of the same resistance is also an extra

advantage. A large diameter. For the same voltage, leads to a lower voltage

gradient at the conductor surface with a tendency of reduced ionisation level of air

and corona.

3. Aluminium Conductor Steel Reinforced (ACSR). ACSR (Aluminium

Conductor Steel Reinforced) conductor comprises hard drawn aluminium wires

stranded around a core of single or multiple strand galvanised steel wire. Fig.

2.1(b) shows an ACSR conductor having 7 strands of steel and 30 strands of

aluminium. Aluminium provides the necessary conductivity while steel provides

the necessary mechanical strength. During manufacture, a layer of grease is put

between aluminium and steel to reduce electrolytic action (corrosion) between

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zinc and aluminium (The steel strands are galvanised with zinc). All transmission

lines and most of the distribution lines use ACSR conductor. These conductors are

manufactured in a wide variety of sizes from 5 mm to over 40 mm overall

diameter.

Aluminum conductor steel reinforced (or ACSR) cable is a specific type of high-

capacity, high-strength stranded cable typically used in overhead power lines. The

outer strands are aluminum, chosen for its excellent conductivity, low weight and

low cost. The center strand is of steel for the strength required to support the

weight without stretching the aluminum due to its ductility. This gives the cable

an overall high tensile strength.

4. Galvanised Steel. Galvanised steel conductors have been used to advantage for

extremely long spans, or for short line selections exposed to normally high

stresses due to climatic conditions. These conductors are found most suitable for

lines supplying rural areas and operating at voltages of about 11 kV, where

cheapness is the main consideration. Iron or steel wire use is most advantageous

for transmission of small power over a short distance, where the size of copper

conductor desirable from economical consideration comes out to be smaller than

SWG, which cannot be used because of poor mechanical strength. This conductor

is not suitable for EHT lines for the purpose of transmitting large amounts of

power over a long distance due to its following properties:

(i) Poor conductivity, 13% that of copper.

(ii) High internal reactance.

(iii) It is subjected to eddy current and hysteresis.

5. Cadmium Copper. The conductor being used in certain cases is copper alloyed

with cadmium. Addition of 1 or 2 % of cadmium in copper increase the tensile

strength by about 40% and reduces the conductivity only by 17% below that of

pure copper. However, cadmium copper is costlier than the pure copper. Use of

cadmium copper will be economical for a line with long spans and small cross-

section i.e. where the cost of conductor material is comparatively small in

comparison to that of supports etc. Cadmium-copper conductors are also

employed for telephone and telegraph lines where currents involved are quite

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small. However, owing to scarcity of copper, cadmium-copper conductors on

communication lines are being replaced by ACSR conductors.

6. Copper-clad Steel. A composite wire, known as copper-clad or copper-weld steel

wire, is obtained by welding a copper coating on a steel wire core. Line

conductors made of copper-clad steel are preferable stranded, and have a

considerably large tensile strength than the equivalent all-copper conductors. The

proportion of copper and steel is so chosen that the conductivity of composite wire

is 30% to 40% of that of copper conductor of equal diameter. Such material

appears to be very suitable for river-crossings or other places where an extremely

long span is involved.

7. Phosphor Bronze. When harmful gases such as ammonia are present in

atmosphere and the spans are extremely long, phosphor bronze is most suitable

material for an overhead line conductor. In this conductor some strands of

phosphor bronze are added to the cadmium copper.

(a) (b)

2.5 INSULATORS

The overhead line conductors are bare and not covered with any insulating

covering/coating. The line conductors are, therefore, secured to the supporting structures

by means of insulating fixtures, called the insulators, in order that there is no current

leakage to the earth through the supports. Insulators are mounted on the cross-arms and

the line conductors are attached to the insulators so as to provide the conductors proper

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Fig no. 2.5 Stranded Conductors .

insulation and also provide necessary clearances between conductors and metal work. The

important properties that an overhead line insulator must possess are:

1. High mechanical strength so as to bear the load due to the weight of line

conductors, wind force and ice loading if any.

2. High relative permittivity so as to provide high dielectric strength.

3. High insulation resistance in order to prevent leakage of currents to earth.

4. High ratio of rupture strength to flash over voltage.

5. Ability to withstand large temperature variations i.e., it should not crack when

subjected to high temperatures during summer and low temperature during winter.

The dielectric strength should remain unaffected under different conditions of

temperature and pressure.

2.5.1 INSULATOR MATERIALS

A true insulator is a material that does not respond to an electric field and completely

resists the flow of electric charge. In practice, however, perfect insulators do not exist.

Therefore, dielectric materials with high dielectric constants are considered insulators. In

insulating materials valence electrons are tightly bonded to their atoms. These materials

are used in electrical equipment as insulators or insulation. Their function is to support or

separate electrical conductors without allowing current through themselves. The term also

refers to insulating supports that attach electric power transmission wires to utility poles

or pylons.

The material most commonly used for overhead line insulators is porcelain but toughened

glass, steatite and special composition materials are also used to a limited extent.

1. Porcelain. Porcelain is produced by firing at a controlled temperature a mixture of

kaolin, feldspar and quartz. It is mechanically stronger than glass. It gives less

trouble from leakage, and is less susceptible to temperature variations and its

surface is not affected by dirt deposits.

On the other hand, it is not so homogeneous as glass, owing to the fact that each

component shell of a porcelain insulator is glazed during manufacturing process

and its satisfactory performance in service depends to a considerable extent on the

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preservation of this glaze which is only of the order of 25 microns in thickness.

Also fault cannot detect easily as it is not transparent. In tension his material is

usually weak and does not withstand tensile stresses exceeding . The

dielectric strength and compressive strength of a mechanically sound porcelain

insulator are about 6.5 kV/mm of its thickness and respectively.

2. Glass. Glass is cheaper than porcelain in the simpler shape and if properly

toughened and annealed gives high resistivity and dielectric strength (14 kV per

mm of thickness of the material). Owing to high dielectric strength, the glass

insulators have simpler design and even one piece design can be used. Glass is

quite homogeneous material and can withstand higher compressive stresses as

compared to porcelain. It has also a lower coefficient of thermal expansion which

minimises the strain due to temperature changes and owing to its transparent

nature flaws in the material can be readily detected by visual examination. The

main disadvantage of the glass is that moisture more readily condenses on its

surface and facilitates the accumulation of dirt deposits, thus giving a high surface

leakage. Also in large sizes the great mass of material combined with the irregular

shape, may result in internal strains after cooling. Glass insulator however, can be

used upto 25 kV under ordinary atmospheric conditions as well upto 50 kV in dry

atmosphere.

3. Steatite. Steatite is a naturally occurring magnesium silicate, usually found

combined with oxides in varying proportions. It has a much higher tensile and

bending stress than porcelain and can advantageously be used at tension towers or

when a transmission line takes a sharp turn.

2.5.2 TYPES OF INSULATORS

Various types of insulators used for overhead transmission and distribution lines are:

1. Pin Type Insulator. A pin insulator is small, simple in construction and cheap. It

is used on lines upto and including 33 kV lines. The conductor is bound into a

groove on the top of the insulator which is cemented on to a galvanised steel pin

attached to the cross arm on the pole or tower. To avoid a direct contact between

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the porcelain and the metal pin, a soft metal (generally lead) thimble is used. An

adequate length of leakage path is obtained by providing the insulator with two or

three petticoats or rain sheds. These are so designed that even when the outer

surface of these insulator is wet due to rain, sufficient leakage resistance is still

given by the inner dry surface. In its electrical behaviour, a pin type insulator may

be compared to a complicated series of conductors with resistances in series and

shunt. The petticoats with the inverting air spaces from the condenser system and

the leakage paths over the surface and through the body of the material are

represented by the resistances.

Fig. no. 2.6 Pin Insulators (a) 11 kV (b) 33 kV

Pin type insulators are used only up to about 33 kV because for higher voltages

they tend to be very heavy and more costly than suspension type insulators.

2. Suspension Type Insulators. The cost of a pin insulators increases very rapidly

with increase in line voltages. Therefore, suspension insulators are used for line

above 33 kV. They are also known as disc insulators or string insulators.

Fig. no. 2.7 Picture of a Suspension Insulator

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A suspension insulator consists of porcelain disc units mounted above the other.

Each disc consists of a single shed of porcelain grooved on the under surface to

increase the creep age distance. The upper surface of each disc is inclined at a

suitable angle to the horizontal in order to ensure free drainage of water. Each disc

is provided with a metal cap at the top and a metal pin underneath. The cap is

recessed so as to take the pin of another unit and thus a string of any required

number of units can be built up. The most commonly used disc is the cemented

cap type.

3. Post Insulators. These are used for supporting the bus bars, and disconnecting

switches in sub-stations. A post insulators is similar to a pin type insulator but has

a metal base and frequently a metal cap so that more than one unit can be mounted

in series. In extra high voltage sub-stations (400 kV and above) polycon post

insulators are used. In this insulator the porcelain elements are in the form of

cones smugly fitting one inside the other and bounded by special cement. The

puncture path is through many layers of porcelain cones and the voltage required

to puncture this path is many times the external flash over voltages so that

insulator is almost puncture proof.

Fig. no. 2.8 Picture of a Post Insulator

4. Strain Insulators. These are special mechanically strong suspension insulators

and are used to take the tension of the conductors at the line terminations and at

positions where there is a change in the direction of line. The discs of a strain

insulator are in a vertical plane as compared to the discs of suspension insulator

which are in a horizontal plane. On extra long spans, viz, at river crossings, two or

three strings of strain insulators, arranged in parallel, are often used.

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Fig. no. 2.9 Picture of a Strain Insulator

The electrical breakdown of an insulator due to excessive voltage can occur in one of two

ways:

Puncture voltage is the voltage across the insulator (when installed in its normal

manner) which causes a breakdown and conduction through the interior of the

insulator. The heat resulting from the puncture arc usually damages the insulator

irreparably.

Flashover voltage is the voltage which causes the air around or along the surface

of the insulator to break down and conduct, causing a 'flashover' arc along the

outside of the insulator. They are usually designed to withstand this without

damage.

Most high voltage insulators are designed with a lower flashover voltage than puncture

voltage, so they will flash over before they puncture, to avoid damage. Dirt, pollution,

salt, and particularly water on the surface of a high voltage insulator can create a

conductive path across it, causing leakage currents and flashovers. The flashover voltage

can be more than 50% lower when the insulator is wet. High voltage insulators for

outdoor use are shaped to maximize the length of the leakage path along the surface from

one end to the other, called the creepage length, to minimize these leakage currents. To

accomplish this surface is molded into a series of corrugations or concentric disk shapes.

These usually include one or more sheds; downward facing cup-shaped surfaces that act

as umbrellas to ensure that the part of the surface leakage path under the 'cup' stays dry in

wet weather. Minimum creep age distances are 20–25 mm/kV, but must be increased in

high pollution or airborne sea-salt areas.

2.6 LINE SUPPORTS

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The function of line support is obviously to support the conductors. Line support must be

capable of carrying the load due to insulator and conductors including the ice and wind

loads on the conductor along with the wind load on the support itself.

The main requirements of the line supports are:

1. High mechanical strength to withstand the weight of conductors and wind

loads etc.

2. Light in weight without the loss of mechanical strength.

3. Cheaper in cost.

4. Low maintenance cost.

5. Longer life.

The choice of line supports for a particular situation depends upon the line span, cross-

sectional area, line voltage, cost and local conditions

Fig. no. 2.10 Picture showing different parameters of a transmission line.

2.6.1 TYPES OF LINE SUPPORTS

The line supports are of various types including wood, steel and reinforced concrete poles

and steel towers either of the rigid or flexible type.

1. Wooden Poles. These supports are cheapest, easily available, provide insulating

properties and therefore, are extensively used for the distribution purposes

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specially in rural electrification keeping the cost low. Their use is usually limited

to low pressures (upto 22kV) and for short spans (upto 60 meters). The wooden

poles well impregnated with creosite oil or any preservative have life from 25 to

30 years. Wooden poles are very elastic and lines employing wooden supports are

often designed throughout for the transverse load. Longitudinal strength at

terminals and for anchor support is provided by means of guys. Double pole

structures of A or H types are often employed for obtaining a higher transverse

strength than that could be economically provided by means of single poles.

Fig. no. 2.11 Picture of a Wooden Pole.

2. RCC Poles. Poles made of reinforced cement concrete (RCC), usually called the

concrete poles, are extensively used for low voltage distribution lines upto 33 kV.

Their construction should conform to the standard specification for RCC work,

but in low case the dimension shall be less 25 cm 25 cm at the bottom and 13cm

13cm at the top. These poles are of two types in shape. One type is square cross-

section from bottom to top. The other type has rectangular bottom and square top

with rectangular holes in it to facilitate the climbing of poles and reduce the

weight of poles. These give good appearance, require no maintenance, have got

insulating properties and resistance against chemical action, very strong, have

longer life and can be used for longer spans (80-200 m). Such poles are most

suitable for water logged situations where other types will not be at all suitable, as

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due to standing water wooden poles will decay very rapidly, and steel construction

will be having deposit of rust. Since these poles are very bulky and heavy,

therefore, transportation cost is heavy and need care in handling and erection.

Fig. no. 2.12 Picture of a RCC Pole.

3. Steel Poles. The steel poles are of three types (i) tubular poles (ii) rail poles and

(iii) rolled steel joists. The tubular poles are of round cross-sections, the rail poles

are of the shape of track used for railways and rolled steel joists are of I cross-

sections. Such poles possess greater mechanical strength and permit use of longer

spans (50-80 m) but cost is higher. Their life is longer than that of wooden poles

and life is increased by regular painting. These poles are set in concrete muffs at

the foundation in order to protect them from chemical action. The average life of

steel poles is more than 40 years.

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Fig. no. 2.13 Picture of a Steel Pole

4. Lattice Steel Towers: The steel tubular poles and concrete poles are usually used

for distribution in urban area to give good appearance and steel rails or narrow-

base, lattice-steel towers are used for transmission at 11 kV and 33 kV and broad-

base lattice-steel towers are used for transmission purposes at 66 kV and above.

The broad-base, lattice-steel towers are mechanically stronger and have got longer

life. Due to their robust construction long spans (300 m or above) can be used and

are much useful for crossing fields, valleys, railways lines, river etc.

Fig. no. 2.14 Picture of a Lattice Steel Tower

CHAPTER-3

DESIGN OF TRANSMISSION LINE

3.1 INTRODUCTION

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The design of a transmission line involves a number of technical and economical aspects.

The power capacity and distance of transmission are specified. The voltage regulation and

efficiency are also specified. The design details include line voltage, size of phase

conductors, span, spacing and configuration of conductors, number and size of earth

wires, number of insulators, clearances, sag under operating and erection conditions, etc.

Once these design features are available, the voltage regulation and efficiency can be

calculated.

3.2 CHOICE OF VOLTAGE

The cost and performance of the line depend, to a great extent, on the line voltage. An

empirical formula for optimum voltage is

V (3.1)

Where V = line voltage in kV

L = distance in km

P = power in kW

A standard voltage nearest to this value should be adopted.

The above formula gives only a preliminary estimate. The choice of the most economical

voltage requires a detailed study of many technical and economical aspects. One a

preliminary estimate is available a detailed analysis is necessary. This becomes all the

more necessary when the final choice is likely to fall in EHV/UHV range.

System Voltages in Transmission Lines

Table shown is the standard system voltages from ANSI standards C84 and C92.2

According to ANSI standards C84 and C92.2, system voltages are recommend to be

within the table shown below. 345kV, 500kV and 765kV are considered to be in the Extra

High Voltage (EHV) level. The choice of system voltage is in the decision of the utility.

22

However, some points needs to be considered in choosing such, like voltage economics,

conductors, distances, equipments, etc.

Table no. 3.1 Standard voltages listed in ANSI standards C84 and C92.2

3.3 SELECTION OF CONDUCTOR SIZE

The cost of conductor size is about 30 to 45 percent of the total cost of the line. Moreover

the cost of towers, foundation and line losses also depend on the conductor size. A proper

selection of the size of phase conductors is, therefore, very important.

Overhead transmission lines invariably use ACSR conductors. These conductors are

manufactured in a variety of sizes (Appendix A).

The size of the conductors should be such that it can carry the rated current continuously

without excessive rise in temperature. The temperature affects the sag and the loss of the

tensile strength (due to annealing) of the conductor. For copper and aluminium, annealing

starts at about and the operating temperature should be well below this value. The

standard practice is to design the line for a conductor temperature of .

The temperature rise of the conductor depends on the conductor heating due to loss

and heat dissipation. In overhead lines heat is dissipated by convection and radiation. A

steady temperature will be reached when

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(3.2)

Where = rms value of conductor current, amperes.

= ac resistance of conductors, ohms/meter length.

= heat loss due to convection, watts per surface area.

= heat loss due to radiation, watts per surface area.

= conductor surface area per meter length.

The heat loss due to convection is given by the equation.

(3.3)

Where p = pressure in atmosphere, Ta is the temperature of air in , v is the velocity of

air in m/sec, d is the diameter of the conductor in m and is the difference between the

temperatures of conductor and air. The above formula is valid is v 0.15 m/sec and d

m.

The heat loss due to radiation is proportional to the difference of the fourth power of the

temperature of the conductor and the surroundings. This loss can be found from the

equation

(3.4)

24

The T1 is the conductor temperature in , T2 is the temperature of surroundings and e

is the relative emissivity of the surface (e = 1.0 for black body and about 0.5 for oxidised

copper).

3.4 CHOICE OF SPAN

A longer span means a smaller number of towers but the towers are taller and more

costly. The higher the operating voltages, the greater should be the span to reduce the

high cost of insulators. Moreover the insulators constitute the weakest part of a

transmission line and a reduction in the number of towers per km (by using longer span)

increases the reliability of the line. For every proposed line there is a definite length of

span which will give the minimum cost of the line. From mechanical consideration there

is a maximum value of span for each conductor size. Many a time it happens that the

conductor size, as determined from electrical calculations is very small and it is possible

to reduce the cost of line by using thicker and stronger conductor so that a longer span

may be employed. Sometimes it is not feasible to determine the tower height and span

length on the basis of the line cost alone because lighting hazards increase greatly as the

height of conductors above ground is increased. Modern high voltage lines have spans

between 200 to 400 m. For river and ravine crossings exceptionally long spans up to 800

m or so have been satisfactory employed.

Specifications

Long span overhead transmission line

Minimum wear

Anti-loose

Well corrosion resistance

Easy installation

Characteristics

25

1. For a single wire material, whether it is damaged or continue, preformed line splicing

section 100% recoverable mechanical strength, and the length of the connecting wire

inside can greatly improve conductivity.

2. For ACSR for repair were not damaged, steel core, wire aluminium wire connecting

section can be restored to 100% strength and 10% of the steel core strength, and the

installation of wires within the article follow, lead performance greatly improved .

3. If the steel core damage, please select the article follow the whole tension.

Table 3.2: The usual spans

With wooden poles 40-50 m

With steel tubular poles 50-80 m

With RCC poles 80-200 m

With steel towers 200-400 m and above

3.5 CHOICE OF CONDUCTORS

Many conductor configurations are used in practice. There is no special advantage in

using symmetrical configuration and in most cases flat horizontal or vertical

configuration are used from mechanical consideration. A flat horizontal configuration

means a lesser tower height but a wider right of way. A vertical configuration means a

taller tower and increased lighting hazards. In spite of these facts, flat horizontal and

vertical configuration is used in many cases. For single circuit lines an L type

configuration is quite popular.

A transmission line may be a single circuit line or double circuit line. A double circuit

line has a higher power transfer capability and greater reliability than a single circuit line.

Each circuit of a double circuit line is usually designed for 75% of the line capacity. In

India, both single circuit and double circuit lines exist in the EHV and high voltage class

(66 kV, 132 kV, 220 kV and 400 kV). In foreign countries also both single and double

circuit line exist. The number of circuits for a proposed line can be determined from the

surge impedance loading (SIL).

3.6 SPACINGS AND CLEARANCES

26

There must be adequate spacing between conductors so that they do not come within

sparking distance of each other even while swinging due to wind. An empirical formula

commonly used for determining the spacing of aluminium conductor lines is

Spacing = meters (3.5)

Where S = Sag in meters

V = Line voltage in kV

Table 3.3: Some typical values of spacing

Line voltage(kV) 0.4 11 33 66 132 220 400

Spacing (m) 0.2 1.2 2.0 2.5 3.5 6.0 11.5

The Indian Electricity Rules specify the minimum clearance between the ground and the

conductor. These values are:

Table 3.4: Minimum clearance between the ground and the conductor

kV 0.4 11 33 66 132 220

Clearance to ground

(a) Across Street (m) 5.8 5.8 6.1 6.1 6.1 7.0

(b) Along Street (m) 5.5 5.5 5.8 6.1 6.1 7.0

(c) Other Areas (m) 4.6 4.6 5.2 5.5 6.1 7.0

These rules also specify the minimum clearance for power lines from buildings, railway

tracks and telecommunication lines, etc.

3.7 INSULATION DESIGN

27

The insulation design affects the performance of the line to a great extent. Line insulation

should be sufficient to take care of switching over voltages, temporary over voltages and

atmospheric over voltages.

The insulation level of the transmission lines is based on the switching surge expectancy

on the system. The maximum switching surge over voltage to the ground is taken as 2.5

p.u and the insulation is designed for this voltage. In addition adequate protection against

atmospheric over voltages (direct lighting strokes) is provided. In EHV and UHV lines

over voltages due to switching surge assume a greater importance than atmospheric over

voltages.

Determination of line insulation:

The insulation of line has to be based upon the consideration or lightning and switching

surges and power frequency over voltages.

With the present day knowledge of lightning behaviour it is possible to build lines to a

certain predetermined level of performance. In case of high voltage lines of 132 kV and

above, these can be made particularly lightning proof by (i) efficient sliding, (ii) low

tower footing impedances. Good shielding is obtained when the shielding angel is about

300 and similarly optimum conditions are generally obtained when the tower-footing

impedance is reduced to about 10 ohms.

The line insulation must be sufficient to prevent a flashover from the power frequency

over-voltage and the switching surges, taking into account all the local unfavourable

circumstances which decrease the flash-over voltage (rain, dust, insulator pollution, etc.).

it is usual to adopt the following over-voltage factors:

Table no. 3.5: Over voltage factors

Switching surge flash-over

voltage

Power frequency flash-

over (wet)

For 220 kV 6.5 Vpn 0.3 Vpn

For 400 kV 5.0 Vpn 3.3 Vpn

28

Where Vpn is the phase to neutral voltage (rms.)

It is a good practice to make an allowance for one or more insulator discs to take care of

the possibility of an insulator unit in the string becoming defective, and also for hot line

maintenance, over and above those required to withstand the above flash-over values.

Accordingly, for lines upto 220kV, one extra disc, and 400 kV lines two extra discs may

be used.

Table no. 3.6 F.O.V. of standard Discs (254×146 mm)

No. Of Discs

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

Dry FOV kV rms.

80

155

215

270

325

380

435

485

540

590

640

690

735

785

830

875

920

965

1010

Wet FOV kV rms.

50

90

130

170

215

255

295

335

375

415

455

490

525

565

600

630

660

690

720

Impulse

FOV(standard full

waves) kV crest

150

255

53

440

525

610

695

780

860

945

1025

1105

1185

1265

1345

1425

1505

1585

1665

29

20

25

30

1055

1280

1505

750

900

1050

1745

2145

2550

In the light of the above discussion, the number of isolator discs of 254×146 mm size

required to withstand switching surge and the power-frequency over- voltage for 132 kV,

220kV, and 400 kV lines is given below:

Table no. 3.7 Recommended Insulation Level for Lines

Normal

system

voltage(kV

)

Vpn

kV

Switching

over-voltage

kV crest

No.

Of

Disc

s

reqd.

Power freq.

Over-

voltage

(wet) kV

No.

Of

Disc

s

reqd.

No. Of Discs

Recommende

d

Employe

d at

present

132

220

400

76

12

7

23

1

76×6.5=495

127×6.5=82

5

231×5=115

5

5

9

13

76×3=228

127×3=381

231×3.3=76

2

6

10

20

7

11

22

9/10

15/16

24

It can be worked out to see that lines working at voltages 132 kV and above are immune

to lightning provided, of course, if proper shielding and low tower footing resistance are

provided. For example, assuming a value of 50 kA (rms.) for the severest lightning

discharge and a tower footing resistance of about 10 ohm, the required impulse strength

of the insulation should be √2×50×103×10 i.e. 700 kV for a line to be immune from

lightning affects. 7discs as recommended in table above for a 132 kV line, would provide

impulse strength of almost (695 kV) the same value (700 kV), still better results in this

case can be obtained by reducing the tower footing resistance. For 132 kV lines the

maximum tower footing resistance kept is 7 ohms.

30

3.8 SELECTION OF GROUND WIRE

The primary function of ground wires is to shield the phase conductors from the lightning

strokes. They are placed above the phase conductors and are grounded at every/alternate

towers. Thus they help in dissipating the lightning currents to the ground.

The selection of the number and configuration of the ground wires is of great importance

in the protection of transmission line against direct strokes. The number of ground wires

may be one or two. A shielding angle of about 30 is considered to be adequate for high

voltage lines. However, for high voltages lines in areas with low lightning hazards,

shielding angle up to 45 have been used. EHV lines are usually provided with two

ground wires and the shielding angle for such lines is kept at about 20 . To prevent back

flashover from the earthed metal to the phase conductors, the tower footing resistance

should not exceed 10 ohms. The vertical separation between the ground wires and phase

conductors should be greater at mid span than at the supports, i.e., the ground wire should

have lesser sage as compared to the phase conductors. The material most commonly used

for ground wires is galvanised steel.

A ground wire should be able to carry the maximum expected lightning current, without

undue heating. It should also have sufficient mechanical strength. Experience has shown

that if a ground wire is mechanically strong, it can carry the maximum, it can carry the

maximum lightning current without excessive heating. Therefore, the size of ground wire

is generally decided on the basis of mechanical strength.

3.9 EVALUATION OF LINE PERFORMANCE

The line parameters are used to evaluate the efficiency and regulation. It is sufficiently

accurate to represent the line by a nominal or circuit for the efficiency and regulation

calculations. However, if the line is very long, the calculations should be based on ABCD

constants. If the efficiency and regulations are not within the prescribed values, it may be

necessary to revise the design by selecting a thick conductor cross-section and changing

31

the conductor configuration. In some cases it may be necessary to use a higher

transmission voltage in the revised design.

3.10 HEIGHT OF TOWER

Number of insulation strings = x

Height of one string = h1

Total height of insulation strings = x × h1

Minimum clearance between the ground and the conductor = h2

Height of tower above insulation strings up to ground wire = h3

Total tower height = (x×h1) + h2 + h3 (3.6)

3.11 LOSSES IN TRANSMISSION LINES

Total transmission line losses can be broken down into three relevant parts namely;

conductor losses, dielectric heating & radiation losses, and coupling & corona losses.

Conductor Losses:

Conductor losses is also popularly known as line heating losses since electric current

that passes through a conductor releases heat. It is known that any metallic materials

possess inherent resistive nature that is why it is inevitable that during electrical flow

through these materials unavoidable power loss occurs. Typical transmission line

conductors consist of resistance that is uniformly distributed throughout the system; as a

result it is safe to say that the total power loss in the line is directly proportional to the

square of the current that passes and the total resistance of the wire. In addition to that,

resistance of the wire is inversely proportional to the diameter of the conductor thus, the

bigger the wire diameter, the lower resistance it can give.

The discussion of transmission lines so far has not directly addressed LINE LOSSES;

actually some line losses occur in all lines. Line losses may be any of three types -

COPPER, DIELECTRIC, and RADIATION or INDUCTION LOSSES.

NOTE: Transmission lines are sometimes referred to as rf lines. In this text the terms are

used interchangeably.

Copper Losses

32

One type of copper loss is I2R LOSS. In rf lines the resistance of the conductors is never

equal to zero. Whenever current flows through one of these conductors, some energy is

dissipated in the form of heat. This heat loss is a POWER LOSS. With copper braid,

which has a resistance higher than solid tubing, this power loss is higher.

Another type of copper loss is due to SKIN EFFECT. When dc flows through a

conductor, the movement of electrons through the conductor's cross section is uniform.

The situation is somewhat different when ac is applied. The expanding and collapsing

fields about each electron encircle other electrons. This phenomenon, called SELF

INDUCTION, retards the movement of the encircled electrons. The flux density at the

center is so great that electron movement at this point is reduced. As frequency is

increased, the opposition to the flow of current in the center of the wire increases. Current

in the center of the wire becomes smaller and most of the electron flow is on the wire

surface. When the frequency applied is 100 megahertz or higher, the electron movement

in the center is so small that the center of the wire could be removed without any

noticeable effect on current. You should be able to see that the effective cross-sectional

area decreases as the frequency increases. Since resistance is inversely proportional to the

cross-sectional area, the resistance will increase as the frequency is increased. Also, since

power loss increases as resistance increases, power losses increase with an increase in

frequency because of skin effect.

Dielectric Losses

DIELECTRIC LOSSES result from the heating effect on the dielectric material between

the conductors. Power from the source is used in heating the dielectric. The heat produced

is dissipated into the surrounding medium. When there is no potential difference between

two conductors, the atoms in the dielectric material between them are normal and the

orbits of the electrons are circular. When there is a potential difference between two

conductors, the orbits of the electrons change. The excessive negative charge on one

conductor repels electrons on the dielectric toward the positive conductor and thus

distorts the orbits of the electrons. A change in the path of electrons requires more energy,

introducing a power loss.

The atomic structure of rubber is more difficult to distort than the structure of some other

dielectric materials. The atoms of materials, such as polyethylene, distort easily.

33

Therefore, polyethylene is often used as a dielectric because less power is consumed

when its electron orbits are distorted.

Radiation and Induction Losses

RADIATION and INDUCTION LOSSES are similar in that both are caused by the fields

surrounding the conductors. Induction losses occur when the electromagnetic field about

a conductor cuts through any nearby metallic object and a current is induced in that

object. As a result, power is dissipated in the object and is lost.

Radiation losses occur because some magnetic lines of force about a conductor do not

return to the conductor when the cycle alternates. These lines of force are projected into

space as radiation and this results in power losses. That is, power is supplied by the

source, but is not available to the load.

Corona loss

Corona as defined by IEEE standard 539-1990

Power lost due to corona process. On overhead power lines, this loss is expressed in watts

per meter (W/m) or kilowatts per kilometre (kW/km). A luminous discharge due to

ionization of the air surrounding an electrode caused by a voltage gradient exceeding a

certain critical value is called corona.

What is Corona Effect?

One of the phenomena associated with all energized electrical devices, including high-

voltage transmission lines, is corona. The localized electric field near a conductor can be

sufficiently concentrated to ionize air close to the conductors. This can result in a partial

discharge of electrical energy called a corona discharge, or corona.

What is Corona?

Electric transmission lines can generate a small amount of sound energy as a

result of corona.

Corona is a phenomenon associated with all transmission lines. Under certain

conditions, the localized electric field near energized components and conductors

34

can produce a tiny electric discharge or corona that causes the surrounding air

molecules to ionize, or undergo a slight localized change of electric charge.

Utility companies try to reduce the amount of corona because in addition to the

low levels of noise that result, corona is a power loss, and in extreme cases, it can

damage system components over time.

Corona occurs on all types of transmission lines, but it becomes more noticeable

at higher voltages (345 kV and higher). Under fair weather conditions, the audible

noise from corona is minor and rarely noticed.

During wet and humid conditions, water drops collect on the conductors and

increase corona activity. Under these conditions, a crackling or humming sound

may be heard in the immediate vicinity of the line.

Corona results in a power loss. Power losses like corona result in operating

inefficiencies and increase the cost of service for all ratepayers; a major concern

in transmission line design is the reduction of losses.

Source of Corona:

The amount of corona produced by a transmission line is a function of the voltage

of the line, the diameter of the conductors, the locations of the conductors in

relation to each other, the elevation of the line above sea level, the condition of the

conductors and hardware, and the local weather conditions

The electric field gradient is greatest at the surface of the conductor. Large-

diameter conductors have lower electric field gradients at the conductor surface

and, hence, lower corona than smaller conductors, everything else being equal.

Irregularities (such as nicks and scrapes on the conductor surface or sharp edges

on suspension hardware) concentrate the electric field at these locations and thus

increase the electric field gradient and the resulting corona at these spots.

Corona also increases at higher elevations where the density of the atmosphere is

less than at sea level. Audible noise will vary with elevation.

Raindrops, snow, fog, hoarfrost, and condensation accumulated on the conductor

surface are also sources of surface irregularities that can increase corona.

35

However, during wet weather, the number of these sources increases (for instance

due to rain drops standing on the conductor) and corona effects are therefore

greater.

Corona produced on a transmission line can be reduced by the design of the

transmission line and the selection of hardware and conductors used for the

construction of the line.

Physical Parameters of Corona:

Corona is caused by the ionization of the media (air) surrounding the electrode

(conductor)

Corona onset is a function of voltage

Corona onset is a function of relative air density

Corona onset is a function of relative humidity

Methods to reduce Corona Discharge Effect:

1. By minimizing the voltage stress and electric field gradient.: This is

accomplished by using utilizing good high voltage design practices, i.e.,

maximizing the distance between conductors that have large voltage differentials,

using conductors with large radii, and avoiding parts that have sharp points or

sharp edges.

2. Surface Treatments: Corona inception voltage can sometimes be increased by

using a surface treatment, such as a semiconductor layer, high voltage putty or

corona dope.

3. Homogenous Insulators: Use a good, homogeneous insulator. Void free solids,

such as properly prepared silicone and epoxy potting materials work well.

4. If you are limited to using air as your insulator, then you are left with geometry

as the critical parameter. Finally, ensure that steps are taken to reduce or eliminate

unwanted voltage transients, which can cause corona to start.

5. Using Bundled Conductors: on our 345 kV lines, we have installed multiple

conductors per phase. This is a common way of increasing the effective diameter

36

of the conductor, which in turn results in less resistance, which in turn reduces

losses.

6. Elimination of sharp points: electric charges tend to form on sharp points;

therefore when practicable we strive to eliminate sharp points on transmission line

components.

7. Using Corona rings: On certain new 345 kV structures, we are now installing

corona rings. These rings have smooth round surfaces which are designed to

distribute charge across a wider area, thereby reducing the electric field and the

resulting corona discharges.

8. Weather: Corona phenomena much worse in foul weather, high altitude

9. New Conductor: New conductors can lead to poor corona performance for a

while.

10. By increasing the spacing between the conductors: Corona Discharge Effect

can be reduced by increasing the clearance spacing between the phases of the

transmission lines. However increase in the phase’s results in heavier metal

supports. Cost and Space requirement increases.

Corona Detection

Light Ultraviolet radiation: Corona can be visible in the form of light, typically a

purple glow, as corona generally consists of micro arcs. Darkening the

environment can help to visualize the corona.

Sound (hissing, or cracking as caused by explosive gas expansions): You can

often hear corona hissing or cracking Sound.

In addition, you can sometimes smell the presence of ozone that was produced by

the corona.

Salts, sometimes seen as white powder deposits on Conductor.

Mechanical erosion of surfaces by ion bombardment

Heat (although generally very little, and primarily in the insulator)

37

Carbon deposits, thereby creating a path for severe arcing

The corona discharges in insulation systems result in voltage transients. These

pulses are superimposed on the applied voltage and may be detected, which is

precisely what corona detection equipment looks for.

Power factor

Power Factor is defined in the fundamentals of electrical engineering as the cosine of the

phase angle between the voltage and the current. An inductive circuit is said to have a

lagging power factor, and a capacitive circuit is said to have a leading power factor

indicate, respectively, whether the current is lagging or leading the applied voltage.

(Stevenson Jr.)

CHAPTER-4

SAMPLE EXAMPLE

Example:

It is proposed to transmit 80 MW at 0.9 power factor lagging over a distance of 150 km.

The line efficiency and regulation at full load should be better than 95% and 10%

respectively. Work out the following details of the transmission line. Make suitable

38

assumptions.

(a) Select line voltage and number of circuits.

(b) Choose proper conductor and span for this line.

(c) Select a suitable value of inter-phase spacing and a suitable configuration of

conductors.

(d) Calculate line parameters. Estimate the line efficiency regulation for full load

condition.

(e) Estimate corona loss.

(f) Find the capacity of shunt compensation equipment to improve the receiving end

power factor to 0.95 lagging.

(g) Estimate line efficiency and regulation for full load at 0.95 power factor lagging.

(h) The line will be erected a temperature of 30°C in still air condition. It is desired that a

factor of safety of 2.5 should be maintained under bad weather condition when the

temperature is 5°C and wind load is 378 N/m2 of projected area. Find the sag and tension

under erection condition. Also find the sag under the bad weather conditions.

(i) Select a suitable number and size of ground wires for this time.

Solution:-

(a) Using Eq. (3.1) the optimum line voltage is,

Where, = Line voltage in kV.

= Distance in km.

39

= Power in kW.

= kV

= 164.43 kV

The nearest standard line voltage is 220 kV. Therefore it should be a 220 kV line. The

surge impedance of a single circuit line is about 400 ohms.

Surge impedance loading (SIL) =

=

= 121 MW

Since the required power transfer is less than SIL, a single circuit is sufficient.

(b) =

= A

= 233.27 A

Let the ambient temperature be . Therefore, temperature rise of can be allowed.

Referring to Appendix A, a suitable conductor for this current is ACSR 6/1/3.66 mm

conductor .

It is necessary to calculate the line losses and the line efficiency to check the suitability of

this conductor. Line losses are approximately equal to where is the total line

resistance per phase .

MINK (ACSR 6/1/3.66 mm) :-

40

For the ACSR 6/1/3.66 mm conductor the resistance at is 0.4565 Ω/km. To

calculate the resistance at we use Eq.,

=

=

=

=

= .56 Ω/km

=

=0.56

= 84 ohms

Line efficiency =

= .85 or 85%

The efficiency is very poor. Hence this conductor size is not suitable.

TIGER (30/7/2.36 mm):-

If we choose the ACSR conductor 30/7/2.36 mm conductor. The resistance of this

conductor at is 0.2220 ohms/km.

=

41

=

= 0.27 ohms/km

R =

= 40.68 ohms

Line efficiency =

=

= 0.9233

= 92.33%

The efficiency is still poor, that shows Tiger is still not a correct selection.

PANTHER (30/7/3.0mm):-

For the ACSR 30/7/3.0 mm conductor the resistance at is 0.140 ohms/km.

=

= 0.171 ohms/km

R =

= 25.65 ohms

Line efficiency =

= 0.95 or 95%

The ACSR conductor 30/7/3.0 mm (PANTHER) has much higher current rating than the

rated current of the purpose line. The line efficiency for this conductor will be higher than

95%

42

Hence the characteristics of this conductor are:

Number of aluminium strands = 30

Diameter of each Al strand = 3.0 mm.

Number of steel strands = 7

Diameter of each steel strand = 3.0 mm

overall diameter = 21 mm

Weight of conductor = 974 kg/km

Ultimate strength = 89.67 kN

Cross section area of Al = 212.1 sq mm

The conditions governing the selection of span has been discussed in section 3.4.

Hence experience has shown that a Span of 300 m is suitable for a 220 kV line.

Minimum clearance between the ground and the conductor is estimated as 7 m using

Table 3.4

Number of insulation strings is calculated as 16 using Table 3.7

Now using Table 3.6 total insulation string length = 0.254 × 16 = 4 m

Hence, total tower height using eq. 3.6 is calculated as 4 + 7 + 6 = 17 m

(c) As per values given in Table 3.3, an inter-phase spacing of 6 meters is suitable for a

220 kV line. The conductor configuration can be horizontal or L-type. Choose horizontal

configuration and 6 meters spacing between adjacent phases (12 meters between the two

phases).

(d) =

43

= 7.5595 m

D = 21 mm

so, r = mm

= 10.5 mm

= m

Geometric mean radius (GMR),

=

= m

= m

Line inductance = mH/km

= mH/km

= 1.36 mH/km

Impedence Z = R+jωL = R+j2πf L (where l = length of transmission line)

= ohms/phase

= 25.65+ j64.1 ohms/phase

= ohms/phase

Line capacitance = µF/km

= µF/km

= μH/km

Y = ωC

44

=

= siemens/phase

Since the length of the line is 150 km, a sufficiently accurate results can be obtained by

the nominal π or T representation. Let we use nominal π representation.

=

=

= (V = voltage of transmission line in volts)

=

= 127017 V

=

= A

Now, using equation

=

=

=137745.97

=

45

=

= 213.31 A

Sending end power factor =

= 0.951 lagging

Sending power factor =

=

= 83.83 MW

Line efficiency =

= 95.43%

at no load = =

= 139560.2533 V

Regulation =

=

=

(e) Taking θ = , Pressure(p) = 74 cm of mercury

And usin

=

= 0.884

Let, = 0.84

46

Using equation,

Kv

= 1.17

Using table given in section 6.4

F = 0.08

Corona loss =

=

= 0.166

The corona loss ( ) of less than 0.2 kW/phase/km is considered to be tolerable. Hence

the corona loss for this line is within limits.

Total corona loss = 0.166

= 74.7 kW

(f) When = lagging

= 0.4843

Receiving end reactive power =

= 38.744 MVar lagging

When = lagging

47

=

Receiving end reactive power =

= 26.296 MVar lagging

Capacity of shunt capacitors to improve the receiving end power factor from 0.9 lagging

to 0.95 lagging =

= 12.448 MVar leading

(g) When the receiving end power factor has been improved to 0.95 lagging.

=

=

= 220.99

= A

= 221 A

=

=

= V

=

= 208.12 A

Sending end power factor =

= 0.985 lagging

Sending end power =

48

= 83.47 83.5 MW

Line efficiency =

= 95.8 %

at no load = =

= 137521.9 137522 V

Regulation =

= 8.27 %

(h) d(overall diameter of conductor) = 21 m

d = 2.1 m

Area A = number of strands

Where r =

= 2.6 sq. M

Now, Young’s Modulus of elasticity E = 91.4

And co-efficient of linear expansion α = 18.44

(Where E and α are constants)

Weight w = 974 kg/km

= w

= 974 = 9.54 N/m

For bad weather conditions (subscript)

49

= 7.938 N/m

=

= 12.41 N/m

=

= 35868 N , = 5 °C

For erection condition (subscript 2)

? ,

Using equation,

Where in N,

Α is area in sq m

α is co-efficient of linear expansion,

E is Young’s Modulus of elasticity in N/m

are temperatures in °C

are forces in N/m

l is the length of span in m

50

now, α A E

= 10645.1613

= 8110.5

Using hit and trial technique,

Sag under erection condition =

Where l = length of span

= .41057 4.11 m

Sag under bad weather condition =

= 3.89 m

Vertical sag under bad weather condition

Vertical sag = S cos γ

51

tan γ =

where = wind load or wind force in N

= p D

Where p = wind pressure

D = diameter of conductor + diameter of ice coating

Since tan γ =

hence

Where w = weight of conductor

= weight of ice

Since there’s no ice hence

γ =

vertical sag = S cos γ m

= S

= 3.89

= 2.99 3 m

CHAPTER-5

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CONCLUSION

In this project we have designed transmission line which comes to be single circuit line

since the required power transfer through a given length is less than SIL (Surge

impedance loading). As per our design requirement the efficiency and regulation of the

line comes within the stipulated limits.

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