Page 923 - Basic Electrical Engineering

Power transmission lines at a national level or even at an international level
are connected through additional paths and lines so that power can be routed
from one power plant to any place where there is need. Surplus power from

one station can be supplied to deficit areas. Such a network allowing
transmission of power nationwide is called a national grid.

Figure 13.6 Shows the schematic diagram of power transmission at high voltage

The power transmitted is proportional to the square of the voltage

. Thus, it is desirable that transmission

voltage be made as high as possible to be able to transmit maximum power

through a line. However, the height of the lines above the ground has to be
increased to keep them away from the ground level. Thus, there is practical

limitation to the level of high-voltage transmission. In India, the highest level
of transmission voltage at present is 400 kV. In some countries the highest

level is 765 kV.

13.8.1 AC Versus DC Transmission

As we have known, P = VI cos Φ. If the voltage level of transmission is

increased, the current level goes down. The cross-sectional area of
transmission line conductors get reduced, and hence the cost gets reduced.

For economical bulk power transmission, the generated voltage level is
stepped up. It has been roughly calculated that most economical transmission