Some Important Notes and Formulas for High Voltage
Transmission Line:
1 Wind Speed
1.1 Basic wind speed
The basic wind speeds have been worked out for 50 years return period. The basic
wind speed is based on peak gust velocity averaged over a short time interval of about
3 seconds and corresponds to 10 m height above mean sea level. The basic wind
speed for wind zone 4 is 47 m/s.
Vb  47 m
sec
1.2 Reference wind speed
It is extreme value of wind speed over an average period of 10 minutes duration and is
to be calculated from basic wind speed by the following relationships:
VR 
Vb
k0
Where k0 is a factor to convert 3 seconds peak gust speed into average speed of wind
during 10 minutes period at a level of 10 meters above ground. k0 is to be taken as
1.375.
1.3 Design wind speed
The design wind speed shall be:
Vd  VR  K1  K 2
Where; VR – reference wind speed in m/sec
K1 – Risk coefficient (for 50 years return period K1 = 1.0)
K2 - Terrain roughness coefficient, as per below table
Table:
Value of K2:
Category 1- Coastal areas; 1.08
Category 2- Normal cross country lines with very few obstacles; 1.0
Category 3- Urban built up areas areas; 0.85
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1.4 Design wind pressure
The design wind pressure on towers, conductors and insulators shall be obtained by
the following relationship:
Pd  0.6  Vd2
where;
Pd = Design wind pressure in N/m2
Vd = Design wind speed in m/sec
1.5 Design wind load on conductor & Groundwire
Fwc  Pd  L  D  Gc  Cdc ,  N 
Where;
Pd
L
=
=
Design wind pressure in N/m2
wind span, being sum of half the span on either side of supporting
point, in m
D
=
Diameter of conductor/groundwire, in m
Gc
=
Gust response factor depends on span, turbulence and height above the
Ground; for 400 m span and terrain category 1 and 40 m heigh tower
the Gc shall be 2.00.
Cdc
=
Drag coefficient, which is 1.0 for conductor and 1.2 for groundwire.
1.6 Design wind load on Tower
FWT  Pd  Cdt  Ae  GT ,  N 
Where;
Pd
=
Design wind pressure in N/m2
Cdt
=
Drag coefficient, which depends on the solidity ratios, 2.0 to 3.6
Ae
=
total net surface area in m2
GT
= Gust response factor
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1.7 Design wind load on Insulators
FWi  1.2  Pd  Ai  Gi,  N 
Where;
Pd
=
Design wind pressure in N/m2
Ai
=
0.5*dia of insulator*length of string in m2
2 Foundation
2.1 Concrete grade
C20 : Characteristic strength of 20 N/mm2
This strength is the minimal design strength, which shall be confirmed from the cube
test.
3 Tower
3.1 Base width
The base width of the tower can be determined from the empirical formula:
B  k M ; cm
Where;
M
K
= Overturning moment, in kg-m
= A constant; 1.93
In general the base width will be:
(1/4 to 1/6 of overall height of the tower)
¼ is for heavy angle tower
1/5 is for medium angle tower
1/6 is for suspension tower
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3.2 Inclination of main legs
Typical inclination of the tower legs (with vertical) for various voltages tower:
Up to 220 kV:
Suspension tower legs (in both directions):
Angle towers (in both directions):
Dead end Towers (in both directions):
40
70
80
400 kV and above:
Suspension tower legs (in both directions):
Angle towers (in both directions):
Dead end Towers (in both directions):
80
100
110
3.3 Minimum Ground Clearance
Voltage Level (kV)
66
132
220
400
Minimum ground Clearance (m)
5.5
6.1
7.0
8.8
4 Construction period
4.1 Tower erection
50 persons per month can erect 80 tonne
4.2 Stringing
50 persons per month will string 2 km double circuit transmission line.
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