2. Content
♦ Conductor and its Types.
♦ Insulators
♦ Line Support
♦ Sag and Tension Calculations
♦ Effect of atmosphere on Transmission Lines
3. Conductor
Properties :
♦ High electrical conductivity.
♦ High tensile strength in order to withstand mechanical stresses.
♦ Low cost so that it can be used for long distances.
♦ Low specific gravity so that weight per unit volume is small.
Types of Conductor :
♦ Copper
♦ Aluminum
♦ Galvanized steel
♦ Cadmium copper
4. Copper
♦ High electrical conductivity
♦ Greater tensile strength
♦ High current density
♦ Smaller cross-sectional area required
♦ High cost & non availability
5. AAC (All Aluminum Conductor)
♦ Cheaper & light in weight
♦ Small conductivity & tensile strength (60% of copper)
♦ Specific gravity lower than copper
♦ Larger cross-arms required
♦ Not suitable for long distance transmission
6. Aluminum Conductor Steel
Reinforced (ACSR)
♦ High-capacity
♦ High-strength stranded conductor
♦ High current carrying capacity.
♦ Low weight
♦ Low cost.
♦ Losses are reduced due to larger
diameter of conductor.
7. Galvanized steel
♦ Very high tensile strength
♦ Long spans
♦ Rural areas
♦ Cheap
♦ Poor conductivity & high resistance
♦ Not suitable for transmitting large power over a long distance
8. Cadmium Copper
♦ Addition of 1% or 2% cadmium to copper
♦ Increased tensile strength by 50% than pure copper
♦ Conductivity reduced by 15% below that of pure copper
♦ Economical for lines of small cross-section due to high cost of cadmium
9. Insulator
Properties:
♦ High mechanical strength
♦ High electrical resistance to avoid leakage currents to earth
♦ Insulator material should be porous , free from impurities & cracks
Types of Insulator:
♦ Pin type.
♦ Suspension type.
♦ Strain type.
♦ Shackle type.
13. Shackle type Insulator
♦ For low voltage distribution lines
♦ Can be used either in a horizontal or vertical position
14. Line Support
Properties:
♦ Cheap in cost and economical to maintain.
♦ Longer Life.
♦ Easy accessibility for maintenances.
♦ High mechanical strength to withstand weight of the conductor.
♦ Light In weight without lose in mechanical strength.
15. Types of Line Support
Choice of supporting structure for a particular case
depends upon the line span, X-sectional area, line voltage, cost
and local conditions.
♦ Wooden poles
♦ Steel poles
♦ R.C.C. poles
♦ Lattice steel towers
16. Wooden poles
Salient feature :
♦ Seasoned wood (sal or chir).
♦ Moderate X-sectional area.
♦ Relatively shorter spans( 50 meters).
♦ Cheap, easily available
♦ Uses in rural areas as an economical proposition.
♦ Comparatively smaller life (20-25 years)
♦ Cannot be used for voltages higher than 20 kV
♦ Less mechanical strength
♦ Require periodical inspection.
17. Steel poles
Salient Feature:
♦ Greater mechanical strength.
♦ Longer life .
♦ Longer spans.
♦ Majorly used in cites.
♦ High-strength conical steel.
♦ Can be used up to 50kv line.
♦ Wooden pools are replaced by steel poles.
18. Reinforced Concrete Poles
(RC poles)
Features:
♦ Greater mechanical strength
♦ Longer life
♦ Longer spans
♦ Good outlook
♦ Little maintenance
♦ Good insulating properties
♦ Moderate Cost
♦ Builded near site to avoid transportation cost
19. Steel towers
Salient Feature:
♦ Long distance transmission.
♦ Higher voltage up from 33KV.
♦ Greater mechanical strength.
♦ Longer life.
♦ Most severe climatic conditions.
♦ Permit the use of longer spans.
♦ Tower footings are usually grounded by driving rods.
♦ High cost.
♦ Greater weight.
♦ Made up of pure steel.
20. Sag and Tension in
transmission lines
Sag :
The difference in level between points
of supports and the lowest point on the
conductor is called sag.
Tension :
The pulling force exerted by each end
of a string is called tension.
21. Relation b/w Sag and Tension
While erecting an overhead line, it is very important that conductors are under
safe tension . If the conductors are too much stretched between supports in a bid to
save conductor material, it causes conductor breakdown due to excessive stress. In
order to permit safe tension in the conductors, they are not fully stretched but are
allowed to have a sag . If sag is greater then it causes physical losses in
transmission line.
The important is that there should be appropriate and balance relation b/w Sag and
tension.
22. Mathematical calculations
There are two ways to calculate sag and tension in transmission lines.
♦ When towers are at equal level.
♦ When towers are at un-equal level.
23. Equal level.
Consider a conductor between two equal level supports A and B with O as the
lowest point. Let assume a point P on the conductor . Taking the lowest point O as
the origin.
l = Length of span
w = Weight per unit length of conductor
T = Tension in the conductor.
Let plot the co-ordinates of point P be x and y.
OP = x
Two forces acting on OP.
♦ The weight Wx of conductor.
♦ The tension T acting at O.
24. Equating both forces
Ty = Wx ∗
x
2
y =
Wx2
T
x = l/2 ► if we using perimeter against A and B.
y = S
Then ,
S =
W l
2
2
2T
S =
W l2
8T
25. Un-Equal level
A conductor suspended between two supports A and B which are at different levels . The
lowest point on the conductor is O.
Let
l = Span length
h = Difference in levels between two supports
x1 = Distance of support at lower level from O
x2 = Distance of support at higher level from O
T = Tension in the conductor
w = Weight per unit length of conductor
27. Evaluating expression 1 and 2
𝑥1 =
1
2
−
𝑇ℎ
𝑊𝑙
And
𝑥2 =
1
2
+
𝑇ℎ
𝑊𝑙
Putting value of x1 and x2
S1 =
W 𝑥12
8T
S2 =
W 𝑥22
8T
28. Effect of atmosphere on
Transmission Lines
Transmission lines have negative effect due to climate changes . some of them are
given.
♦ Wind
♦ Icing
♦ Fog
29. Wind:
Due to wind, transmission lines are badly effected. There is damage on towers,
insulator and also conductors.
Due to sudden wind change weight of Conductors increases .which effects tension
and sag in Conductor.
Icing:
Icing increases weight and diameter of conductor . Which increases tensions on
Conductor.
Due to increase in tension Conductor can be break down . which cause shutdown
of transmissions.
Fog:
Due to fog corona losses occur. Sparking and short circuit happened
due to Fog. Because there is ionized O2 in air .which causes burring
and short circuit.
