transmission lines- presentation
TRANSCRIPT
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SUMBUL.H.ABIDI
ELECTRICAL DEPARTMENT
S.P.C.E.
TRANSMISSION LINE
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Earth
Wire.
Line
Insulator
Line
support
Conductor
Main comp-
Onents of
Over headline
Earth
Wire.
Line
Insulator
Line
support
Conductor
Main comp-
Onents of
Over headline
COMPONENTS OF OVER HEAD
LINE:-
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The line supports are poles and the chief requirements for such supports are :
i ) They must be mechanically strong with factor of safety of 2.5 to 3 .
ii ) They must be light in weight without the loss of strength .
iii ) They must have least number of parts .
iv ) They must be cheap .
v ) Their maintenance cost should be minimum .vi ) They must be easily accessible for point and erection of line conductors .
vii ) They must have longer life .
viii ) They must be of pleasing shape
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The different types of poles which can be used as line supports are :
a. Wooden poles .b. Steel tubular polesc. Reinforced concrete poles .d. Steel towers .
Fig.(1):Single phase single-circuit
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Spacing between the
conduc tors
The most suitable spacing between theconductors can be arrived at bymathematical calculations.
It can only be obtained by empiricalformulae which have been obtained from
practical considerations.
Fig: Three-phase single circuit horizontal disposition ofconductor and steel towers
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Generally the following formulae are used for obtainingspacing between the conductors :
Spacing=150
VS
Where,
S=sag in meters
V=voltage in KV
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Span
SagInsulator
Tension Tower
Supporting
Tower Tension Tower
Transmission Line
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Wooden PolesCheapest
Use for lines where span is short and tension is low.
Pole has limitation of height and diameter.Wood has natural insulating property and lesser flashover
takes place due to lightning.
For greater strength double pole structure of A or H are
used.
Concrete PolesConcrete poles are reinforced to give greater strength.
It has longer life span than that of wood
The maintenance cost is less.
They are heavy and are likely to get damage
duringloading,unloading,transportation and erection due totheir brittle nature.
Nowadays pre-stressed concrete supports are
manufactured in pieces.
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Steel polesTubular steel poles or girder steel masts is favored for low and medium
distribution voltages.Longer span is possible.
Poles need to be galvanized or painted periodically to prevent corrosion.
Maintenance is expensive.
Steel TowersLines of 66 KV are supported on towers.
They fabricated from painted or galvanized angle section which are
transported separately and erection done on site.
Long life and high degree of reliability
They can withstand severe weather conditions.
The height of the towers depend on operating voltage and span length.The leg of the tower are set on concrete foundation
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Types of steel towers
1. Tangent towers They are used for straight runs. Stress is due to weight of line,wind and ice load.
Extra forces due to break in the line on one side.
Base is either square or rectangle.
Suspension type insulators used.
2. Deviation tower They are used where transmission line changes
direction.
They have broader base , stronger members and
are costlier Strain insulators are used.
They are classified
i. Small angle towers(2o-15ochange in direction)
ii. Medium angle towers(15o-30ochange in
direction)
iii. Large angle towers(30o-60ochange in direction)
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Transmission line (Steel tower)
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Transmission line (Steel tower)
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Lattice Towers
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TYPES OF INSULATORS USED IN
OVERHEAD SYSTEM.
Pin type insulator.
Suspension type insulator.
Strain type insulator.
Shackle type insulator.
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PIN INSULATORS
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.
The conductor passes through this groove and is bound by the
annealed wire of the same material as the conductor.
Pin type insulators are used for transmission and distribution of electric
power at voltages up to 33 kV. Beyond operating voltage of 33 kV, the
pin type insulators become too bulky and hence uneconomical.
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Causes of failuresInsulators are required to withstand both mechanical and
electrical stresses.
The electrical stress is primarily due to line voltage and
may cause the breakdown of the insulator. The electrical breakdown of the insulator can occur either
by flash overorpuncture.
In flashover, an arc occurs between the line conductor and
insulator pin (i.e., earth) and the discharge jumps across
the air gaps, following shortest distance. Figure shows the
arcing distance (i.e. a + b + c) for the insulator.In case of flash-over, the insulator will continue to act in its
proper capacity unless extreme heat produced by the arc
destroys the insulator.
In case of puncture, the discharge occurs from conductor
to pin through the body of the insulator. When such
breakdown is involved, the insulator is permanentlydestroyed due to excessive heat. In practice, sufficient
thickness of porcelain is provided in the insulator to avoid
puncture by the line voltage.
The ratio of puncture strength to flashover voltage is
known as safety factor.
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Locking KeyInsulator's Head
Expansion LayerImbedded Sand
SkirtPetticoats
Iron CapBall Socket
CompressionLoading
CementInsulating Glassor Porcelain
BallCorrosion Sleevefor DC Insulators
Steel Pin
Ball and socket type suspension insulator
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Line Voltage
Number of Insulators per
String
69 kV 46
115 kV 79
138 kV 810
230 kV 12
345 kV 18
500 kV 24
765 kV 3035
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ADVANTAGES
Each unit is designed for operating voltage of about 11KV .
The string is free to swing in any direction.It is designed for longer span and higher mechanical loading.
There is decreased liability to lightning disturbance if the
string is suspended from a metallic structure, which work as
lightning shield.
DISADVANTAGES
Spacing between conductor is increased
Height of tower increases
Cost is high.
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Strain or Tension InsulatorStrain or tension insulators are designed for handling mechanical stresses at angle
positions where there is a change in the direction of the line at termination of line.
Suspension insulator are arranged in a horizontal position. They are used to handle
mechanical stresses and take the pressure off a conductor at the end of atransmission line, at a sharp corner or curve or over long river crossings.
Strain insulators are typically used for higher voltage transmissions.
Shackle Type Insulators
Shackle type insulators, similar to strain type insulators, are used on sharp curves,
end poles and in section poles.
However, unlike strain insulators, shackle insulators are designed to support lower
voltages. These insulators are single, round porcelain parts that are mounted
horizontally or vertically.
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V-Strings
A single string of insulator follows the conductor and sways like a pendulum in a
strong side wind.V-strings are used to prevent conductor movement at towers. They are used for high
voltage transmission system.
INSULATOR MATERIAL1. Porcelain with glazed surface
2. Toughened glass
3. Polymer Insulator
Lattice Guyed tower
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