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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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