Electric power can be transmitted or distributed either by means of underground cables or by overhead line. The underground cables are rarely used for power transmission due to two main reasons. Firstly, power is generally transmitted over long distances to load centres. Obviously, the installation costs for underground transmission will be very heavy. Secondly, electric power has to be transmitted at high voltages for economic reasons: It is very difficult to provide proper insulation to the cables to withstand such higher pressures. Therefore, as a rule, power transmission over long distances is carried out by using overhead line. With the growth in power demand and consequent rise in voltage levels, power transmission by overhead lines has assumed considerable importance.
The undergound system is much more expensive than overhead system. Therefore, it has limited use for distribution in congested areas where safety and good appearances are the main considerations.
In overhead lines, bare conductors are used and air acts as the insulation. The necessary insulation between the conductors can be provided by adjusting the spacing between them,
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. In this chapter, we shall focus our attention on the various aspects of mechanical design of overhead lines.
Main Components of Overhead Lines
An overhead line may be used to transmit or distribute electric power. The successful 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:
- 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.
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.