Introduction.doc

SEMINAR REPORT

OVERHEAD TRANSMISSION

SYSTEM

E L E C T R I C A L D E P A R T M E N T

ID: NO: D11EE132

OVERHEAD TRANSMISSON SYSTEM

POSITION OF TRANSMISSON SYSTEM IN POWER SUPPLY SYSTEM:

1. GENERATING STATION:

Power is generated around 11KV to 15KV voltage. Modern generating

stations are of large capacity. Step up transformer is kept in the switch yard

of the generating station, which steps up the generated voltage to the

required voltage of transmission. In the power supply required shown in

figure. a, the voltage is generated at 11KV and it is stepped up to 220KV

Fig: a: diagram of power supply system

ELECTRICAL ENGINEERING DEPARTMENT Page 2


2. PRIMARY TRANSMISSION:

Primary transmission is carried out at 132KV, 220 KV or 400KV.600 KV systems

are also developed and 700KV to 1000KV is under design stage. Single circuit or

double circuit over head transmission line is used. This line is run to different

remote areas. One or more lines are used depending upon the need. A receiving

station is used at the end of the transmission line.66KV voltage is stepped down to

66KV.

3. SECONDARY TRANSMISSION:

Required numbers of 66kv secondary transmission lines are run in different areas.

The secondary transmission is done either the over head lines or by the

underground cables. Distribution substation is kept the end of the secondary

Transmission line.66kv voltage is stepped down to 11kv in the distribution

substation.

4. PRIMARY DISTRIBUTION OR HT DISTRIBUTION:

In distribution substation the 66KV voltage is stepped down to 11KV.HT

consumers are supplied power through 11KV feeder. This is called the primary

Distribution .Other 11KV feeders are run to different area. Distribution

Transformer is kept at the end of each 11KV feeder.

5. SECONDARY DISTRIBUTION OR LT DISTRIBUTION:

Distribution transformer step down the 11KV voltage to 415V.From here the

consumer are supplied power with the help of three Phase wire and one neutral

wire. House hold consumer are given power through one phase and one neutral


OVERHEAD TRANSMISSON SYSTEM wire, while the consumer like flour mills, pump for agriculture etc.are given power

through three phase wire and one neutral wire.

ELEMENTS OF TRANSMISSON SYSTEM:

Overhead a.c Transmission system is widely used so in the element of

Transmission system only the element of the over head a.c Transmission systems

are included. More over use of underground cables for high voltage transmission

line is very limited so it is not discussed here. The following are the element of the

overhead a.c transmission line.

1. Step up transformer:

This transformer is kept in the switch yard of the generating station. This

Transformer step up the generated voltage to the voltage of Transmission

2. Line supports:

Steel tower are used to support the conductor of the transmission line.

3. Line insulators:

Suspension and strain type porcelain insulator are used to insulate the conductor

from the ground.

4. Conductor:

Copper conductors were used for the transmission line but ACSR conductors are

now widely used in place of copper conductor due to the increased price of copper.

5. Step down transformer:


OVERHEAD TRANSMISSON SYSTEM Step down transformer is provided at the end of the transmission line which steps

down the primary transmission voltage.

6. Protective devices:

For the protection of the equipment of the transmission circuit breakers, earth

switch lightning arresters, ground wires, arcing horns etc.are used.

7. Regulators:

Voltage regulators are provided for the voltage regulation of line and for the

improvement of the power factor of the line, the synchronous condensers are

provided at the end of the transmission line.

TWO TYPES OF TRANSMISSON SYSTEM:

1. OVERHEAD TRANSMISSON SYSTEM:

In overhead Transmission system, the conductor should be run above the ground

from one place to the other. Line supports are used for this. Insulator are kept on

the



Fig: b: overhead transmission line

Cross arms fitted to the poles or steel towers. And live wires are tied to the

insulator. Line supports are made of wood, R.C.C. or steel. Here we shall obtain


http://en.wikipedia.org/wiki/File:Pylon_ds.jpg

OVERHEAD TRANSMISSON SYSTEM information about construction of different types of line supports and their relative

advantages and disadvantages.

CLASSIFICATION OF OVERHEAD TRANSMISSION

LINES:

The overhead transmission lines are classified as:

I) Short transmission lines.

When the length of an overhead transmission line is up to 50km and the line

voltage is comparatively low (<20kv), it is usually considered as a short

Transmission line. Due to smaller length and lower voltage, the capacitance effects

are small and hence can be neglected.therefor, while studying performance of a

short transmission line, only resistance and inductance of the line are taken into

account.

II) Medium transmission lines.

When the length of an overhead transmission line is about 50-150km and the line

voltage is modernly high (>20kv<100kv), it is considered as a medium

transmission line. Due to sufficient length and voltage of the line, the capacitance

effects are taken into accont.for purposes of calculation, the distributed

capacitance of the line is divided and lumped in the form of condensers shunted

across the line at one or more points.



III) Long transmission lines.

When the length of an overhead transmission line is more than 150 km and line is

very high (>100kv), it is considered as along transmission line. For the treatment

of such a line, the lines constant are considered uniformly distributed over the

whole length of the line and rigorous methods are employed for solution.

MAIN COMPONENTS OF OVERHEAD LINES:

An overhead line may be used to transmit or distribute electrical power. The

successful operation line depends to a great extent upon the mechanical design of

the line. While construct an overhead line, it should be ensured that mechanical

strength of the line is such so as to provide against the most probable weather

condition. In general, the main components of an overhead line are

(1).conductor: Conductor which carry electric power from the sending end

station to the receiving end station.

(2). Supports:

Supports which may be poles or towers and keep the conductor from at a suitable

level above the ground.

(3).insulator:

Insulator which are attached to supports and insulate the conductors from the

ground.

Fig: c: insulator



4).cross arms:

Cross arms which provide support to the insulators.

Fig: d: cross arms

TYPES OF LINE SUPPORT OF OVERHEAD LINE:

The supports used for transmission and distribution of electric power of various

types including wooden poles, steel poles, R.C.C.pole and lattice steel towers. The

choice of supporting structure for a particular case depends upon the line span, X-

sectional area, line voltage, cost and local condition.

1: Wooden poles:

These are made of seasoned wood (Sal or chir) and are suitable for lines of

moderate X-sectional are and of relatively shorter spans, say up to 50 meters. Such

supports are cheap, easily available, provide insulating properties and, therefore,


OVERHEAD TRANSMISSON SYSTEM are widely used for distribution purposes in rural areas as an economical

proposition. The wooden poles generally tend to rot below the ground level,

causing foundation failure. In order to prevent this, the portion of the pole below

the ground level is impregnated with preservative compounds like creosote oil.

Double pole structure of the “a” or “h” type are often used to obtain higher

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

Fig: e: wooden pole

The main objections to wooden support are:

1): tendency to rot below the ground level.

2): comparatively smaller life (20-25years.)

3): cannot be used for voltages higher than 20 KV.

4): less mechanical strength and

5): require periodical inspection.



Fig: f: wooden pole

2: Steel poles:

The steel poles are often used as a substitute for wooden poles. They possess

greater mechanical strength, longer life and permit longer spans to be used. Such

poles are generally used for distribution purposes in the cities. This type of

supports need to be galvanized or painted in order to prolong its life. Steel poles

are two types (1): rail pole and (2): tubular pole.

In tubular poles hollow round steel pipes are used. It is in three to four steps over

the length. It is broad at the lower portion and narrow at the above. And steel poles

can be used for the street lighting by bending the upper portion.



Fig: g: steel poles

3: Rcc pole:

The reinforced concrete poles have become very popular as line supports in recent

years. They have greater mechanical strength, longer life and permit longer spans

than steel poles. Moreover, they give good outlook; require little maintaince and

have good insulating properties.fig show R.C.C poles for single and double circuit.

The holes in the poles facilities the climbing of poles and at the same time reduce

the weight of line supports.

The main difficulty with the use of these poles is the high cost of transport owing

to their heavy weight. Therefore such poles are often manufactured at the site in

order to avoid heavy cost of transportation.

Due to the unavailability of the wood poles and increasing cost of the steel poles,

RCC poles or pre stressed concrete poles are becoming popular. Its mechanical

strength is more and is used for more than span length. It does not require


OVERHEAD TRANSMISSON SYSTEM maintaince as required in wood pole and steel pole. It is used in low voltage and

high voltage distribution lines.

Fig: h: RCC poles

4: Steel tower:

In practice, wooden, steel and reinforced concrete poles are used for distribute

purposes at low voltage, say up to 11kv.however, for long distance transmission at

higher voltage, steel tower are invariably employed. Steel tower have greater

mechanical strength are invariably employed. Steel tower have greater mechanical

strength, longer life, can withstand most severe climatic condition and permit the

use of longer spans. The risk of interrupted service due to broken or punctured

insulation is considerably reduced owing to longer spans. Tower footings are

usually grounded by driving rods into the earth. This minimizes the lightning

troubles as each tower acts as a lightning conductor.

Types of insulator:


OVERHEAD TRANSMISSON SYSTEM The successful operation of an overhead line depends to a considerable extent

upon the proper selection of insulator. There are several types of insulator but the

most commonly used are pin type, suspension type strain insulator and shackle

insulator.

1:-pin type insulator:

The part section of a pin type insulator is shown in fig. As the name suggest, the

pin type insulator is secured to the cross-arm on the pole. There is a groove on the

Upper end of the insulator for housing the conductor passes through this groove

and is bound by the annealed wire of the same material as the conductor.

Fig: i: pin type insulator

Pin type insulators are used for transmission and distribution of electric power at

the voltage up to “33kv” the pin type insulator become too bulky and hence

uneconomical.

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

of the insulator. When such breakdown is involved, tice, sufficient thickness of

porcelain is provided in the insulator to avoid puncture by the line voltage. The

ratio of puncture strength to flash over voltage is known as safety factor. It is


OVERHEAD TRANSMISSON SYSTEM desirable that the value of safety factor is high so that flash over takes place

before the insulator gets punctured. For pin type insulator the value of safety factor

is about 10.

Safety factor of insulator = puncture strength

__________________

Flash –over voltage

2: suspension type insulator:

The cost of pin type insulator increases rapidly as the working voltage is

increased.therefor, this type of insulator is not economical beyond 33kv.for high

voltage (>33kv), it is usual practice to use suspension type insulator shown in fig.

They consist of porcelain discs connected in series by metal links in the form of a

string. The conductor is suspended at the bottom end of this string while the other

end of the string is secured to the cross arm of the tower. Each unit or discs is

designed for low voltage, say 11kv.the number of discs in series would obviously

depend upon the working voltage. For instances, if the working voltage is 66kv,

then six discs in series will be provided on the string.



Fig: j: suspension insulator.

3: strain insulator:

When there is a dead end of the line or there is corner or sharp curve, the line is

subjected to greater tension. In order to relive the line of excessive, strain insulator

are used. For low voltage lines (<11kv), shackle insulator are used as strain

insulator. However for high voltage transmission lines, strain insulator consists of

an assembly of suspension insulator as shown in fig. The discs of insulator are

used in the vertical plane. When the tension in lines is exceedingly high, as at long

river spans, two or more strings are used in parallel.

Fig: strain insulator.


OVERHEAD TRANSMISSON SYSTEM 4: shackle insulator:

In early days, the shackle insulators were used as strain insulator. But now days

they are frequently used for low voltage distribution lines. Such insulator can be

used either in horizontal position or in a vertical position, they can be directly

fixed to the pole with a bolt or to the cross arm.fig show a shackle insulator fixed

to the pole. The conductor in the groove is fixed with a soft binding wire.

Fig: k: shackle insulator

Corona:

When an alternating potential difference is applied across two conductors whose

spacing is large as compared to their diameters, there is no apparent change in the

condition of atmosphere air surrounding the wires if the applied voltage is low.

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

disruptive voltage the conductor are surrounded by a faint violet glow called

corona.



The phenomenon of corona is accompanied by a hissing sound, production of

ozone, power loss and radio interference, the higher the voltage is raised, the

larger and higher the luminous envelope becomes, and greater are the sound, the

power loss and the radio noise. If the applied voltage is increased to breakdown

value flash-over will occur between the conductors due to the breakdown of air

insulation.

The phenomenon of violet glow, hissing noise and production of ozone gas in an

overhead transmission line is known as corona.

If the conductors are polished and smooth the corona glow will be uniform

Throughout the length of the conductor, otherwise the rough points will appear

brighter. With d.c.voltage.there is difference in the appearance of the two wires.

The positive wire has uniform glow about it, while the negative conductor has

spotty glow.

Fig: L: corona effect.

Theory of corona formation some ionization is always present in air due to

cosmic rays, ultraviolet radiation and radioactivity. Therefore, under normal

condition, the air around the conductor contains some ionized particles and neutral

molecules. When p.d is applied between the conductor potential gradient is setup

in the air which will have maximum value at the conductor surfaces. Under the



influence of potential gradient the existing free electron acquire velocities. The

greater the applied voltage, the greater the potential gradient and more is the

velocity of free electrons.

When the potential gradient at the conductor surface reaches about 30kv per cm

(max.value).the velocity acquired by the free electron is sufficient to strike a

neutral molecule with enough force to dislodge one or more electron from it. This

produces another ion and one or more free electron which is turn are accelerated

until they collide with other molecule, thus producing other ions. Thus the process

of ionization is that either corona is formed or spark takes place between the

conductors.

Skin effect:

When a conductor is carrying steady direct current (d.c.),this current is uniformly

distributed over the whole x-section of the conductor .however an alternating

current flowing through the conductor does not distribute uniformly rather it has

the tendency to concentrate near the surface of the conductor as shown in fig. This

is known as skin effect.

The tendency of alternating current to concentrate near the surface of a conductor

is known as skin effect.



Due to skin effect, the effective area of cross-section of the conductor through

which current flows in reduced. Consequently the resistance of the conductor is

slightly increased when carrying an alternating current. The cause of skin effect

can be easily explained. a solid conductor may be thought to be consisting of a

large number of strand each carrying a small part of the current. The inductance of

each strand will vary according to its position. Thus, the strands near the center are

Surrounded by a greater magnetic flux and hence larger inductance than that near

The surface. The high reactance of inner strand causes the alternating current to

flow near the surface of conductor. This crowding of current near the conductor

surface is the skin effect.



The skin effect depends upon the following factor:

(i) nature of material

(ii) Diameter of wire: - increase with the diameter of wire.

(iii) Frequency: - increase with the increase in frequency.

(iv) Shape of wire: - less for stranded conductor than the solid conductor.

It may be noted that skin effect is negligible when the supply frequency is

low (<50Hz) and conductor diameter is small (<1cm).

COMPARISION BETWEEN THE OVERHEAD SYSTEM AND UNDERGROUND SYSTEM:

BASE OVERHEAD SYSTEM

UNDERGROUND SYSTEM

1. Appearance. Appearance is not good Wires etc.are not seen so



due to overhead wires, insulator, cross arms etc. (dis adv.)

the appearance is good.(adv.)

2. Maintenance. Maintenance is more as the wires, insulator etc.are in open.(dis.adv)

Maintenance is less as the cable is under the ground.

3. Possibility of fault.

As the conductors are in open, possibility of occurring fault is more. (Dis adv.)

Possibility of occurring fault is comparatively less as the cable is under the ground (adv.)

4. Safety. Safety is less as the wire may break and cause harm to the living beings. (Dis adv.)

There is more safety (adv.)

5. Possibility of lightning stroke.

More (dis adv.) Less (adv.)

6. Voltage drop. As the spacing of the wires is more, the inductance is more so voltage drop is more due to the inductive reactance. (Dis adv.)

Inductance is less due to the closed conductor, so the voltage drop is less due to the inductive reactance (adv.)

7. Disturbance to the nearby communication line.

Possibility is more. Transposition of line is done to reduce it.(dis adv.)

No disturbance is caused (adv.)

8. Fault location and repairing.

Fault location is easy and repairing is also easy.(adv.)

Fault location is not easy and repairing is difficult.

9. Capital cost. Overall cost is less even through the cost of supports is more (adv.)

Capital cost is more (dis adv.)

10.Working voltage. Overhead lines are used up to and more than 400 kv(adv.)

Not used for high working voltage (dis adv.)

11.Flexibility More. Changes are easy (adv.)

Making change is difficult (dis adv.)

CONCLUSION:



Our discussion answers the most important questions about the behavior of

transmission lines. We show that it is the attenuation of higher frequencies that

causes the rounding of voltage transitions as they propagate along a transmission

line. Higher frequencies travel faster than lower frequencies, so dispersion cannot

be the cause of the rounding. Very low frequencies travel several times slower

than high frequencies, and the result is that the low-frequency content of a voltage

transition might arrive several microseconds later than the initial sharp edge. This

late-arrival explains the settling of transitions that we observe at the far end of

long transmission lines.

REFERANCES:

Www.wikipedia.org/wiki/Electric_power_ transmission

Www.intel.com/education/.../class06_ transmission _ line _basics

www.amanogawa.com/ transmission .html

http://indolinkenglish.wordpress.com

http://www.velco.com/Environment/Documents/Transmission%20Line


http://www.velco.com/Environment/Documents/Transmission%20Line

http://indolinkenglish.wordpress.com/

http://www.amanogawa.com/transmission.html

http://Www.intel.com/education/.../class06_transmission_line_basics

http://Www.wikipedia.org/wiki/Electric_power_transmission

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