Transmission System 1

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viewers ,After electricity is producedat power plants it has to get to thecustomers that use the electricity. Our

cities, towns, states and the entire

country are criss-crossed with powerlines that "carry" the electricity.

As large generators

spin, they produce electricity with a

voltage of about 11,000 volts.

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The electricity first goes to a

transformer at the power plant that

boosts the voltage up to 400,000 volts.

When electricity travels long distances

it is better to have it at higher

voltages.

Another way of saying this is that

electricity can be transferred more

efficiently at high voltages.

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The long thick cables of transmission

lines are made of copper or aluminumbecause they have a low resistance. We

know that the higher the resistance of a

wire, the warmer it gets. So, some of the

electrical energy is lost because it is

changed into heat energy. High voltage

transmission lines carry electricity long

distances to a substation.

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Electrical power is generated at different

generating stations. These generating

stations are not necessarily situated at the

load center. During construction of

generating station number of factors to be

considered from economical point of view.

These all factors may not be easily

available at load center, hence generatingstation are not normally situated far away

from load center.

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Fundamentally there are two systems by

which electrical energy can be transmitted(1) High Voltage DC Electrical Transmission

System

(2) High voltage AC Electrical TransmissionSystem

There are some advantages in using DC

transmission system ,in India we are usingHigh voltage ACElectrical Transmission

System

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Only two conductor are required for Dctransmission system.It is further possible to use

only one conductor of DC transmission system if

earth is utilized as return path of the system

ii) The potential stress on the insulator of DC

transmission system is about 70% of samevoltage AC transmission system. Hence less

insulation cost is involved in DC transmission

system.iii)Inductance,capacitance,phase displacement

and surge problems can be eliminated in DC

system.8


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Even having these advantages in DC system,

generally electrical energy is transmitted bythree(3) phase AC transmission system.

1-The alternating voltages can easily be

stepped up by using a step up transformerwhich is called as power transformer too. &

down by using power transformer and

distribution transformer both, which is notpossible in DC transmission system.

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iv) AC transmission system is more likely

to be affected by corona than DC

system.v) Construction of AC electrical power

transmission network is more

completed than DC system.vi) Proper synchronizing is required

before inter connecting two or more

transmission lines together,

Synchronizing can totally be omitted in

DCtransmission system.12


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Now days the electrical power demand is

increasing very rapidly. For fulfilling these

huge power demands the modern time

requires creation of bigger and bigger power

generating stations. These power generating

stations may be hydro

electric, thermal or

atomic. Depending upon the availability of

resources these stations are constructed

different places. These places may not be

nearer to load centers where the actual

consumption of power takes place.13


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So it is necessary to transmit these huge

power blocks from generating station to

their load centers. So it is necessary to

transmit these huge power blocks from

generating station to their load centers.

Long and high voltage transmission

networks are needed for this

purpose. Distribution of electrical poweris done at lower voltage levels as specified

by consumers.

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The main functions of a substation are:

the transfer of power in a controlled

manner as well as to make it possible toperform the necessary switching

operations in the grid(energizing and de-

energizing of equipment and lines) andprovide the necessary monitoring,

protection and control of the circuits

under its control and supervision. Asubstation is a high-voltage electric system

facility. It issued to switch generators,.15


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Equipment, and circuits or lines in and

out of a system. It is also used to change

AC voltages from one level to another,

and/or change alternating current to

direct current or direct current to

alternating current. Some substations

are small with little more than a

transformer and associated switches.

Others are very large with several

transformers and dozens of switches and

other equipment16


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Step up Substation

Step up substations are associated with

generating stations. Generation ofpower is limited to low voltage levels

due to limitations of the rotating

alternators. These generating voltagesmust be stepped up for economical

transmission of power over long

distance. So there must be a step upsubstation associated with generating

station.17


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Primary Step down Substation

The primary step down sub stations are

created nearer to load center along theprimary transmission lines. Here

primary transmission voltages are

stepped down to different suitablevoltages for secondary transmission

purpose. We make use of step down

transformer for this purpose so that

power transformer

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Secondary Step down SubstationAlong the secondary transmission lines,

at load center, the secondary

transmission voltages are furtherstepped down for primary distribution

purpose. The stepping down of

secondary transmission voltages toprimary distribution levels are done at

secondary step down substation.

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Outdoor type sub station

Outdoor type Substation areconstructed in open air. Nearly all 66kv

132KV, 220KV, 400KV substation are

outdoor type substation.Although now days special GIS (Gas

Insulated Sub Station) are constructed

for Extra High Voltage system which aregenerally situated under roof.they

occupy much space

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Outdoor substations

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Indoor Substation

The substations are constructed underroof is called indoor type substation.

Generally 11KV and sometime 33KV

substation are of this type. these typesof substation occupy less space as

compare to out door substation these

types of substation serves peak loadsometimes

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Indoor Substation

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Underground Substation

The substation are

situated at underground is called

underground substation. In congestedplaces where place for constructing

distribution substation is difficult to find

out,.

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Pole mounted substation

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A Pole Mounted Sub-station is a large,

free standing, outdoor electricalequipment that is mostly located in

residential places. Its main purpose is

to step-down the lethal 11kV to415/240V for light, commercial and

residential loads (consumers).these are

commonly seeing substation.generellyThis is a step down transformer with

some accessories.

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Pole mounted substations


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The 11kV line is connected to the Step-

Down Transformer (11kV/415V) though

a gang isolator and fuses. The lighting

arrestors are installed on the H.T side to

protect the Sub-Station from lightening

strokes. The transformer steps down

the 11kV to 415V, 3phase, 4-wire

supply. The voltage between any two

lines is 415V, and between any line and

a phase is 240V.

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The Step-Down Transformer is oil cooled

and its this oil that people steal. This costs

the electricity board millions of rupees

annually. Please be warned that the

current flowing through the transformer isdeadly, it would kill you in seconds.

Stealing that oil means that the

transformer will overheat, reduce itsefficiency and possible explode under

normal circumstances.

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The Step-Down Transformer


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Extra High voltage transmission line

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http://www.gophoto.it/view.php?i=http://4.bp.blogspot.com/_7dhDrlYW2uE/S_q4jo18PKI/AAAAAAAAAbk/VWV0MW8LZC8/s1600/400kV.jpg


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Extra High Voltage Systems - 400kV Lines

Nominal Voltage 400kV

Circuits per phase 1

Sub-conductors per phase 2

Span 400m

Conductor Name and Size Moose,54/3.53mm

Aluminum 7/3.53mm Steel

Sub-conductor Diameter 3.177cm

Bundle Configuration Horizontal

Bundle Spacing 450mmInter phase Spacing 11m

Conductor Configuration Horizontal

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Resistance per phase per km at 20degrees= 0.0274Ohm

Inductive Reactance per phase per km = 0.3321Ohm

Shunt Admittance per phase per km = 3.2983*10-6Surge Impedance = 282Ohm

Surge Impedance Loading = 505MW

Current Carrying capacity at 40degrees ambient

temperature = 900AGround Wire 2Number,7/3.66mm of

Galvanized Steel

Ground wire height at tower = 30.4mTower height = 30.4m

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Transmission system

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This pictures shows the full scheme of

transmission system including primary

transmission

And secondary transmission from higher

voltage to lower voltage for the consumer For

domestic and commercial use in steps

1-extra high voltage

2-high voltage (220,132 kv)

3-66kv,33kv4-11kv/415v

5-domestic and commercial use

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Electricity is generated in powerstations and from there it istransmitted across the country bypower cables to towns and villages.As you know, all cables have

resistance and so as the electricitypasses through them it will loseenergy as heat in the cable.

The energy lost in a power cable thatis carrying a current I and is ofresistance R is given by the formula:

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Power loss (W) = I

2

R


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As you can see the power loss depends

on the resistance of the cable and so toreduce this power loss the resistance of

the cable must be as small as possible.

The resistance of a given length ofcable can be reduced by:

(a) using thicker cables

but this willmake them too heavy;

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(b) using material which is a better

electrical conductor such as silver

much too expensive.

(c) cooling the cables with a jacketof liquid hydrogen too expensive.So changing the resistance is not apractical idea, but the current

passing through the cable can bealtered using a transformer

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To keep the power loss as small as

possible the transmitted voltage is veryhigh. Electricity generated in a power

station at 11 000 V (11 kV) is stepped up

to 220 kV or 400 kV for transmissionacross large distances. Near towns,

villages and industrial sites there are

transformers that step down the voltageready for use.

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A transmission lines has the following accessories

1-tower

2-cross arms

3-dampers

4-spacers

5-conductors6-insulators

7-ground wire

8-arching horn gap arrestors7-bird guard

8-danger plate--------etc.

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CLOSER LOOK OF DAMPER

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(e) Low specific

Gravity.

(d) Low

cost

(C) Should not

Be brittle

(b) High tensile

Strength

(a)High

Conductivity

CHARECTERISTICS OF CONDUCTING MATERIAL:-


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ACSRconductor used in overhead

lines

1 2

3

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Documents

Transmission System 1