When suspension string is used to sustain extraordinary tensile load of conductor it is referred as string insulator. When there is a dead end or there is a sharp corner in transmission line, thline has to sustain a great tensile load of conductor or strain. A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties.

1. Mechanical Design of
Transmission Lines

2. General Considerations
Electrical Considerations for T.L. Design:
• Low voltage drop
• Minimum power loss for high efficiency of
power transmission.
• The line should have sufficient current
carrying capacity so that the power can be
transmitted without excessive voltage drop
or overheating.

3. • Conductivity of Conductor:
R = ρ.L/A , or
R = L/Ϭ. A
Where:
L: Conductor length.
A: Conductor cross sectional area.
ρ: resistivity
Ϭ: Conductivity (Ϭ= 1/ρ)

4. • The conductor conductivity must be very high
to reduce Conductor resistance R and hence
reduce losses
PL= 3 I2 .R

5. Mechanical Considerations for T.L. Design:
• The conductors and line supports should
have sufficient mechanical strength:
- to withstand conductor weight, Conductor
Tension and weather conditions (wind, ice).
- The Spans between the towers can be long.
- Sag will be small.
- Reducing the number and height of towers
and the number of insulators.

7. • Heat expansion coefficient must be very small.
Rt = R0. (1 + α0 .t)
αt = α0/(1+ α0.t)
α t is the heat expansion coefficient at t.

8. TYPES OF CONDUCTORS
MATERIALS

9. 1- All Aluminum Conductors (AAC)
lowest cost – low mechanical strength
Used for small span

12. 2- Aluminum Conductor Steel
Reinforced (ACSR)
1- Steel strands
2- Aluminum strands
ACSR (26/7)

17. Advantages of ACSR
• High mechanical strength can be utilized by
using spans of larger lengths.
• A reduction in the number of supports also
include reduction in insulators and the risk of
lines outage due to flash over or faults is
reduced.
• losses are reduced due to larger diameter of
conductor.
• High current carrying capacity.

18. 3- All Aluminum Alloy Conductor
(AAAC)

20. 4-Aluminum Conductor Alloy
Reinforced (ACAR)

21. Types of Supports
• Wooden Poles
• Reinforced Concrete Poles
• Steel Poles
• Lattice Structure Steel Towers

22. Wooden Poles

27. Reinforced Concrete Poles

30. Steel Poles

31. Types of Towers
1- Suspension Tower
2- Tension Tower
3- Angle Tower
4- End Tower

32. 1- Suspension Tower

34. 2- Tension Tower

36. 3- Angle Tower

37. 4- End Tower
This type of towers exists in the beginning and
at the end of the line which exposed to
tension in one side.

38. SAG AND TENSION
CALCULATIONS

39. Sag of Transmission Lines
Sag of T.L depends on:
- Conductor weight.
- Span length,
- Tension in the conductor, T
- Weather conditions (wind , ice).
- Temperature.

40. Minimum Clearance between the
ground and the conductor
kV C (m)
0.4 5.5
11 5.5
33 6.0
66 6.2
132 6.2
220 7.0
400 8.4

41. Conductor Spacing
Spacing = (S )0.5 + V/150
Where:
S: Sag in meters.
V: Line voltage in kV.