Electrical Power Systems

388 Electrical Power Systems
where
Le = Lavg + 2
3 (Lmax – Lavg ) ...(15.68)
L avg = average span in line
= 1
n
n
Li i= 1
å ...(15.69)
L max = maximum span in line
= max[L 1, L 2, L 3, ..., L n] ...(15.70)
The tension of line T can be calculated using this equivalent span length and expression for
sag is detined as
15.6 EECT O ICE
e 2
d = wL
8T
...(15.71)
The sag is determined for the span design at which the transmission line is constructed so that
accumulations of snow or ice and excessive temperature changes will not stress the conductor
beyond its elastic limit, may cause permanent stretch or fatigue failures from continued
vibrations. In mountaneons areas, the thickness of the ice formed on the conductor is very
significant. Accumulations of ice on the line conductor has the following effects on the line
design:
1. Increase the dead weight per meter of the line
2. Increase the projected surface of the line subject to wind pressure.
More likely configuration of a conductor with ice coating is shown in ig. 15.7. However, for
the sake of simplicity, it can be assumed that ice coating is uniform over the surface of the
conductor, as shown in ig. 15.8.
ig. 15.7: More likely configuration of cross-sectional area of ice covered conductor.