A power system is a combination of central generating stations, power transmission system, Distribution and utilization system. Electric power is produced at the power stations which are located at favourable places, generally quite away from the consumers. It is then transmitted over large distances to load centres with the help of conductors known as transmission lines. Finally, it is distributed to a large number ofsmall and big consumers through a distribution network,
Energy
is generated (transformed from one to another) at the generating stations.
Generating stations are of different type, for example, thermal, hydro, solar
power stations, nuclear. The generated electricity is stepped up through the
transformer and then transferred over transmission lines to the load centres.
Electric
power is generated at a voltage of 11 to 25 kV which then is stepped up to the
transmission levels in the range of 66 to 400 kV (or higher). As the
transmission capability of a line is proportional to the square of its voltage,
research is continuously being carried out to raise transmission voltages. Some
of the countries are already employing 765 kV. The voltages are expected to
rise to 800 kV in the near future. In India, several 400 kV lines are already
in operation. One 800 kV line has just been built. . The transmission of
electric power at high voltages has several advantages including the saving of
conductor material and high transmission efficiency. It may appear advisable to
use the highest possible voltage for transmission of electric power to save
conductor material and have other advantages. But there is a limit to which
this voltage can be increased. It is because the increase in transmission
voltage introduces insulation problems as well as the cost of switchgear and
transformer equipment is increased. Therefore, the choice of proper
transmission voltage is essentially a question of economics. Generally, the
primary transmission is carried at 66 kV, 132 kV, 220 kV or 400 kV.
Transmission System
And Distribution System
The large network
of conductors between the power station and the consumers broadly divided into
two parts viz., can be transmission system and distribution system. Each part
can be further subdivided into two — primary transmission and secondary
transmission and primary distribution and secondary distribution.
Primary
transmission.
The first stepdown
of voltage from transmission level is at the bulk power substation, where the
reduction is to a range of 33 to 132 kV, depending on the transmission line voltage.
The electric power at 132 kV is transmitted by 3-phase, 3-wire overhead system
to the outskirts of the city. This forms the primary transmission.
Secondary
transmission
The primary
transmission line terminates at the receiving station (RS) which usually lies
at the outskirts of the city. At the receiving station, the voltage is reduced
to 33kV by step-down transformers. From this station, electric power is
transmitted at 33kV by 3-phase, 3-wire overhead system to various sub-stations
(SS) located at the strategic points in the city. This forms the secondary
transmission.
Primary
distribution
The secondary
transmission line terminates at the sub-station (SS) where voltage is reduced
from 33 kV to 11kV, 3-phase, 3-wire. The 11 kV lines run along the important
road sides of the city. This forms the primary distribution. It may be noted
that big consumers (having demand more than 50 kW) are generally supplied power
at 11 kV for further handling with their own sub-stations.
Secondary
distribution
In the last stage
in a Power System, the electric power from primary distribution line (11 kV) is
delivered to distribution sub-stations (DS) or Distribution Transformer. A
typical pole mounted distribution transformer is shown in Fig. 5. These
sub-stations are located near the consumers’ localities and step down the
voltage to 400 V, 3-phase, 4-wire for secondary distribution. The voltage
between any two phases is 400 V and between any phase and neutral is 230 V. The
single-phase residential lighting load is connected between any one phase and
neutral.
