Transmission lines should be able to the transmit power over the required distance economically and satisfy the electrical and mechanical requirements prescribed in particular cases. It is necessary to transmit a certain amount of power at a given power factor, over a given distance and be within the limits of the given voltage regulation, efficiency and losses.
The lines should stand the weather conditions of the locality in which they are laid. This involves the wind pressure and the temperature variation at the location and the lines should also be designed for the corresponding mechanical loading.
The voltage regulation will give us the voltage drop between the sending-end and the receiving-end in transmission line at no-load and full load condition. Because of very high voltage possibility of corona formation and the losses due to corona are another consideration. The charging current of the line depends on the capacity of the line and should not exceed the limit. As far as the general requirement of Transmission lines are concerned, over the line should have enough capacity to transmit the required power, should maintain continuous supply without failure and should be mechanically strong, so that there are no failures due to mechanical breakdown also.
Long transmission lines are required to transmit power from hydro-electric power stations to the places of load, as the sites for water power would be far away from the load centers. Transmission lines required to transmit power from thermal stations or nuclear power stations to the load centers may be short lines compared to the lines from hydro-electric projects. Transmission lines are also required for interconnecting power systems having a number of power stations for transfer of power or energy from one system to the other at the time of peak load for outage due to fault conditions. The transmission system consists of all the equipment from generating-end, switchyard lines, switching stations, equipment at receiving-end till the entrance to sub-transmission circuits, to distribution circuits.