Transmission System

Report
Presentation on
ByEr Rajinder Kumar
Lecturer Electrical Engg.
Govt polytechnic college
Amritsar
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viewers ,After electricity is produced
at power plants it has to get to the
customers that use the electricity. Our
cities, towns, states and the entire
country are criss-crossed with power
lines 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 aluminum
because 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 generating
station 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 Transmission
System
There are some advantages in using DC
transmission system ,in India we are using
High voltage AC Electrical Transmission
System
.
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Only two conductor are required for Dc
transmission 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 same
voltage 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.
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Even having these advantages in DC system,
generally electrical energy is transmitted by
three(3) phase AC transmission system.
1-The alternating voltages can easily be
stepped up by using a step up transformer
which is called as power transformer too. &
down by using power transformer and
distribution transformer both, which is not
possible in DC transmission system.
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ii) Maintenance of AC substation is quite
easy and economical compared to DC
system.
iii) The transforming in AC sub station is
much easier than motor - generator sets
in DC system
iii) AC substation system is simple to
adopt as compare to DC system.
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AC transmission system also have some
disadvantages like,
i) The volume of conductor used in AC
system is much higher than that of DC
ii)The reactance of the line, affects
the voltage regulation of electrical
power transmission system
iii) problems of skin effects and proximity
effects only found in AC 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
DC transmission system.
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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.
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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 power
is 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 to
perform
the
necessary
switching
operations in the grid(energizing and deenergizing of equipment and lines) and
provide the necessary monitoring,
protection and control of the circuits
under its control and supervision. A
substation is a high-voltage electric system
facility. It issued to switch generators,.
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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 equipment
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Step up Substation
Step up substations are associated with
generating stations. Generation of
power is limited to low voltage levels
due to limitations of the rotating
alternators. These generating voltages
must be stepped up for economical
transmission of power over long
distance. So there must be a step up
substation associated with generating
station.
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step down Substation
The stepped up voltages must be
stepped down at load centers, to
different voltage levels for different
purposes. Depending upon these
purposes the step down substation are
further categorized in different sub
categories.
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Primary Step down Substation
The primary step down sub stations are
created nearer to load center along the
primary transmission lines. Here
primary transmission voltages are
stepped down to different suitable
voltages 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 Substation
Along the secondary transmission lines,
at load center, the secondary
transmission voltages are further
stepped down for primary distribution
purpose. The stepping down of
secondary transmission voltages to
primary distribution levels are done at
secondary step down substation.
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Outdoor type sub – station
Outdoor
type
Substation
are
constructed 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 are
generally situated under roof.they
occupy much space
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Outdoor substations
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Indoor Substation
The substations are constructed under
roof is called indoor type substation.
Generally 11KV and sometime 33KV
substation are of this type. these types
of substation occupy less space as
compare to out door substation these
types of substation serves peak load
sometimes
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Indoor Substation
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Underground Substation
The
substation
are
situated at underground is called
underground substation. In congested
places 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 electrical
equipment that is mostly located in
residential places. Its main purpose is
to step-down the lethal 11kV to
415/240V for light, commercial and
residential loads (consumers).these are
commonly seeing substation.generelly
This 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 StepDown 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 Oil Circuit Breaker (O.C.B), installed on
the L.T side automatically isolates the
transformer from the consumers in case
of any fault. The Pole Mounted Substations are generally used for transformer
capacity up to 200kVA They should be
periodically checked for dielectric strength
of oil in the transformer and (OCB) In case
of repair the transformer or (O.C.B) both
the Gang Isolators and (O.C.B) should be
shut off.
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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 is
deadly, it would kill you in seconds.
Stealing that oil means that the
transformer will overheat, reduce its
efficiency 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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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
450mm
Inter 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*10-6
Surge Impedance
=
282Ohm
Surge Impedance Loading
=
505MW
Current Carrying capacity at 40degrees ambient
temperature
=
900A
Ground Wire
2Number,7/3.66mm of
Galvanized Steel
Ground wire height at tower
=
30.4m
Tower height
= 30.4m
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Conductor Height at tower
= 20.75m
Conductor sag at 0degrees
= 8.39m
Ground wire sag at 0degrees
= 6.654m
Shielding
angle
= 10degrees
Suspension string
23dics,225*145mm
Impulse Flashover voltage
=
1550kV
Examples:
Sultanpur-Lucknow Line,
Obra-kanput Line,
Kanpur-Marad nagar Line.
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Have you ever wondered why high
voltage power lines buzz (zszszs sound)?
Its because they are producing corona
which is interacting with the
surrounding
air.
Corona is the process by which current,
perhaps sustained, develops from an
electrode with a high potential in a
neutral fluid, usually air, by ionizing that
fluid so as to create plasma around the
electrode.
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The ions generated eventually pass charge
to nearby areas of lower potential, or
recombine to form neutral gas molecules.
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CORONA
FACTORS AFFECTING ON CORONA LOSS:-
Factors affecting on
corona loss.
(1)Condition on the (2) Potential difference
Atmosphere.
Between two conductor.
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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,33kv
4-11kv/415v
5-domestic and commercial use
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Electricity is generated in power
stations and from there it is
transmitted across the country by
power cables to towns and villages.
As you know, all cables have
resistance and so as the electricity
passes through them it will lose
energy as heat in the cable.
The energy lost in a power cable that
is carrying a current I and is of
resistance R is given by the formula:
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2
Power loss (W) = I R
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As you can see the power loss depends
on the resistance of the cable and so to
reduce this power loss the resistance of
the cable must be as small as possible.
The resistance of a given length of
cable can be reduced by:
(a) using thicker cables — but this will
make 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 jacket
of liquid hydrogen — too expensive.
So changing the resistance is not a
practical idea, but the current
passing through the cable can be
altered using a transformer
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To keep the power loss as small as
possible the transmitted voltage is very
high. Electricity generated in a power
station at 11 000 V (11 kV) is stepped up
to 220 kV or 400 kV for transmission
across large distances. Near towns,
villages and industrial sites there are
transformers that step down the voltage
ready for use.
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A transmission lines has the following accessories
1-tower
2-cross arms
3-dampers
4-spacers
5-conductors
6-insulators
7-ground wire
8-arching horn gap arrestors
7-bird guard
8-danger plate--------etc.
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CLOSER LOOK OF DAMPER
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CHARECTERISTICS OF CONDUCTING MATERIAL:-
(a) High
(b) High tensile
Conductivity
Strength
(e) Low specific
Gravity.
(C) Should not
Be brittle
(d) Low
cost
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ACSR—conductor used in overhead lines
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[email protected]
9914253131
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