MECHANICAL DESIGN OF
TRANSMISSION LINE
The successful operation of an overhead line
depends to a great extent upon the mechanical
design of the line
An overhead line is subjected to uncertain weather
conditions and other external interferences
mechanical strength of the line is such so as to
provide against the most probable weather
conditions
MAIN COMPONENTS OF OVERHEAD
LINES
Conductors which carry electric power from the
sending end station to the receiving end station.
Supports which may be poles or towers and keep
the conductors at a suitable level above the ground.
Insulators which are attached to supports and
insulate the conductors from the ground.
Cross arms which provide support to the insulators.
Miscellaneous items such as phase plates, danger
plates, lightning arrestors, anti-climbing wires etc.
CONDUCTOR MATERIALS
Properties of conductor material:
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.
STRANDED CONDUCTORS
For n layers, the total number of individual wires is 3n(n + 1) + 1
Commonly used conductor materials:
Copper
Aluminium
Steel-cored aluminium
Galvanized steel
Cadmium copper
Line Supports:
Supporting structures for overhead line conductors
are various types of poles and towers called line
supports.
Properties of line supports are,
– High mechanical strength to withstand the weight
of conductors and wind loads etc.
– Light in weight without the loss of mechanical
strength.
– Cheap in cost and economical to maintain.
– Longer life.
– Easy accessibility of conductors for maintenance.
VARIOUS TYPES OF LINE SUPPORTS
Wooden poles
Steel poles
R.C.C. Poles
Lattice steel towers.
INSULATORS
provide necessary insulation between line
conductors and supports
Prevent leakage current from conductors to earth
Properties:
High mechanical strength in order to withstand conductor load, wind load etc.
High electrical resistance of insulator material in order to avoid leakage currents to
earth.
High relative permittivity of insulator material in order that dielectric strength is high.
The insulator material should be non-porous, free from impurities and cracks otherwise
the permittivity will be lowered.
High ratio of puncture strength to flashover
Types of Insulators
Pin type insulators
voltages upto 33 kV
Suspension type insulators
Strain insulators
Shackle insulators
SAG IN OVERHEAD LINES
“The difference in level between points
of supports and the lowest point on
the conductor”
Calculation of Sag
When supports are at equal levels
l = Length of span Two Forces:
w = Weight per unit length of The weight wx of
conductor conductor acting at a
T = Tension in the conductor distance x/2 from O.
The tension T acting at O.
Equating the moments of above two forces about point O,
When supports are at equal levels
l = Span length
h = Difference in levels between two supports
x1 = Distance of support at lower level (i.e., A) from O
x2 = Distance of support at higher level (i.e. B) from O
T = Tension in the conductor
Also, x1 + x2 1
=l
2
Effect of wind and ice loading
Total weight of conductor per unit length is
When the conductor has wind and ice loading also,
the following points may be noted :
The conductor sets itself in a plane at an angle θ to the
vertical where,
The sag in the conductor is given by ,
The vertical sag = S cos θ
NUMERICAL PROBLEMS
1. An overhead line has a span of 260 m, the weight of
the line conductor is 0·68 kg per meter run. Calculate
the maximum sag in the line. The maximum allowable
tension in the line is 1550 kg. Ans: 3·7 m`
2. A 132 kV transmission line has the following data :
Wt. of conductor = 680 kg/km ; Length of span =
260 m
Ultimate strength = 3100 kg ; Safety factor = 2
Calculate the height above ground at which the
conductor should be supported. Ground clearance
3. A transmission line conductor is supported from two
towers at heights of 70m above water level. The horizontal
distance between the towers is 300 m. If the tension in the
conductors is 1500 kg, find the clearance at a point mid-
way between the towers. The size of the conductor is 0·9
cm2 and density of conductor material is 8·9 gm/cm3.
Ans: 64 m
4. A transmission line has a span of 150 m between level
supports. The conductor has a cross-sectional area of 2
cm2. The tension in the conductor is 2000 kg. If the specific
gravity of the conductor material is 9·9 gm/cm3 and wind
pressure is 1·5 kg/m length, calculate the sag. What is the
vertical sag? Ans: S=3·48 m ; Vertical Sag=2·77 m
