An overhead line is subjected to uncertain weather conditions and other external interferences.This calls for the use of proper mechanical factors of saftey in order to ensure the continuity of operation in the line. In general, the strength of the line should be such so as to provide against the worst probable weather conditions.
Main Components of Overhead Lines
An overhead line may be used to transmit or distribute electric power. The succcessful operation of an overhead line depends to a great extent upon the mechanical design of the line. While constructing an overhead line, it should be ensured that mechanical strength of the line is such so as to provide against the most probable weather conditions. In general, the main components of an overhead line are:
(i) Conductors which carry electric power from the sending end station to the receiving end
station.
(ii) Supports which may be poles or towers and keep the conductors at a suitable level above the
ground.
(iii) Insulators which are attached to supports and insulate the conductors from the ground.
(iv) Cross arms which provide support to the insulators.
(v) Miscellaneous items such as phase plates, danger plates, lightning arrestors, anti-climbing
wires etc.
The continuity of operation in the overhead line depends upon the judicious choice of above components. Therefore, it is profitable to have detailed discussion on them.
Conductor Materials
The conductor is one of the important items as most of the capital outlay is invested for it. Therefore, proper choice of material and size of the conductor is of considerable importance. The conductor
material used for transmission and distribution of electric power should have the following properties :
(i) high electrical conductivity.
(ii) high tensile strength in order to withstand mechanical stresses.
(iii) low cost so that it can be used for long distances.
(iv) low specific gravity so that weight per unit volume is small.
All above requirements are not found in a single material. Therefore, while selecting a conductor material for a particular case, a compromise is made between the cost and the required electrical and mechanical properties. Commonly used conductor materials. The most commonly used conductor materials for overhead lines are copper, aluminium, steel-cored aluminium, galvanised steel and cadmium copper.
The choice of a particular material will depend upon the cost, the required electrical and mechanical properties and the local conditions. All conductors used for overhead lines are preferably stranded* in order to increase the flexibility. In stranded conductors, there is generally one central wire and round this, successive layers of
wires containing 6, 12, 18, 24 ...... wires. Thus, if there are n layers, the total number of individual
wires is 3n(n + 1) + 1. In the manufacture of stranded conductors, the consecutive layers of wires are
twisted or spiralled in opposite directions so that layers are bound together.
Copper. Copper is an ideal material for overhead lines owing to its high electrical conductivity and greater tensile strength. It is always used in the hard drawn form as stranded conductor.
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