From Transmission and Distribution Electrical Engineering, Third Edition
Overhead lines are, in essence, air-insulated cables suspended from insulated supports with a power transfer capacity approximately proportional to the square of the line voltage. Overhead lines are cheaper in initial capital cost and are generally more economic than cable feeders. For the transmission of equivalent power at 11 kV a cable feeder would cost some 5 times the cost of a transmission line, at 132 kV 8 times and at 400 kV 23 times. Such comparisons must, however, be treated in more depth since they must take into account rights of way, amenity, clearance problems, planning permissions associated with the unsightly nature of erecting bare conductors in rural and urban areas, and ongoing maintenance requirements.
18.2 ENVIRONMENTAL CONDITIONS
In order to match both the mechanical and electrical characteristics of the overhead line conductor to the environmental conditions climatic details must first be collected and analysed. The parameters required are as described in Chapter 17, Section 17.2.1.
The maximum, minimum and average ambient temperature influences conductor current rating and sag. For temperate conditions typically 20 C with 55 C temperature rise. For tropical conditions 35 C or 40 C with 40 C or 35 C temperature rise. Maximum conductor operating temperature should not exceed 75 C for bare conductors to prevent annealing of aluminium. Conductor temperatures up to 210 C are possible with GAP conductor (see Section 18.3.2).
Required for structure and conductor design. Electrical conductor ratings may be based on cross wind speeds of 0.5 m/s...
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