Page 924 - Basic Electrical Engineering

voltage is 1 kV per 1.6 km or 1 kV per mile. Thus, the most economical
transmission voltage for a distance of around 600 km, works out around 400
kV. In India, the transmission voltage varies from 66 kV to 400 kV. For

transmission of power to distances more than 600 km, it has been calculated
that dc transmission becomes more economical than ac transmission.

Electricity is generated by using an alternator, i.e., an ac generator at 11 kV
or more. For ac transmission, the three-phase lines from the generator are

connected to the primary side of a step-up transformer. The secondary
terminals are connected to the transmission lines.

For dc transmission, the alternating voltage is first converted into dc using
an ac to dc converter, and is then connected to the transmission lines. At the
receiving end of the transmission lines dc is converted to ac and distributed to

the consumers through distribution lines, step-down transformers and
underground cables. It may be mentioned here that transmission of power is

done in two stages. In the first stage, the voltage level may be 220 kV and in
the second stage the voltage level is brought down to 132 kV or less. The

level of transmission may be brought down even to 33 kV. The first stage of
transmission is called primary transmission while the second stage of

transmission, at voltages ranging from 132 kV to 33 kV, is called secondary
transmission.

13.8.2 Distribution System

The distribution voltage is either 11 kV or 415/230 V. At the distribution

substation, power is received from the secondary transmission lines at a
voltage of, say 33 kV. The voltage is further stepped down to 11 kV by the

distribution transformer installed at the distribution substation. Heavy
industries are supplied with 11 kV or even higher voltages, who in turn step

down the voltage using their own transformers. For other consumers
electricity is supplied at 415/230 V. Three-phase supply is provided at 415 V

and single-phase supply is made at 230 V.
Thus, the part of power system which provides electricity to the customers
is called the distribution system as shown in Fig. 13.7.