1. TRANSMISSION LINE
SUMBUL.H.ABIDI
ELECTRICAL DEPARTMENT
S.P.C.E.

2. COMPONENTS OF OVER HEAD
LINE:-
Conductor
Main comp-
Earth Onents of Line
Wire. Over head support
line
Line
Insulator

3. 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

4. The different types of poles which can be used as line supports are :
a. Wooden poles .
b. Steel tubular poles
c. Reinforced concrete poles .
d. Steel towers .
Fig.(1):Single phase single-circuit

5. Spacing between the
conductors
 The most suitable spacing between the
conductors can be arrived at by
mathematical calculations.
 It can only be obtained by empirical
formulae which have been obtained from
practical considerations.
Fig: Three-phase single circuit horizontal disposition of
conductor and steel towers

6. Generally the following formulae are used for obtaining
spacing between the conductors :
V
Spacing= S
150
Where,
S=sag in meters
V=voltage in KV

7. Transmission Line
Span Supporting
Tower Tension Tower
Tension Tower
Insulator
Sag

8. Wooden Poles
•Cheapest
•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 Poles
•Concrete 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 to
their brittle nature.
•Nowadays pre-stressed concrete supports are
manufactured in pieces.

9. Steel poles
•Tubular 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 Towers
•Lines 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

10. 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)

12. Transmission line (Steel tower)

13. Transmission line (Steel tower)

14. Lattice Towers

15. TYPES OF INSULATORS USED IN
OVERHEAD SYSTEM.
• Pin type insulator.
• Suspension type insulator.
• Strain type insulator.
• Shackle type insulator.

16. 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.

17. Causes of failures
•Insulators 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 over or puncture.
•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 permanently
destroyed 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 .

19. Ball and socket type suspension insulator
Iron Cap
Locking Key
Ball Socket
Insulator's Head
Compression
Expansion Layer Loading
Imbedded Sand Cement
Insulating Glass
or Porcelain
Skirt
Petticoats Steel Pin
Ball
Corrosion Sleeve
for DC Insulators

20. Number of Insulators per
String
Line Voltage
69 kV 4–6
115 kV 7–9
138 kV 8–10
230 kV 12
345 kV 18
500 kV 24
765 kV 30–35

21. 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.

22. Strain or Tension Insulator
•Strain 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 a
transmission 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.

23. 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 MATERIAL
1. Porcelain with glazed surface
2. Toughened glass
3. Polymer Insulator
Lattice Guyed tower