Electric Supply System

The conveyance of electric power from a power station to consumers’ premises is known as electric supply system. An electric supply system consists of three principal components viz., the power station, the transmission lines and the distribution system.

The electric supply system can be broadly classified into

1. d.c. or a.c. system
2. overhead or underground system.

Now-adays 3-phase, 3-wire a.c. system is universally adopted for generation and transmission of electric power as an economical proposition.

Typical a.c. Power Supply Scheme

Electric Supply Schematic

The network can be broadly divided into two parts viz.,

1. transmission system and
2. distribution system.

Each part can be further sub-divided into two—primary transmission and secondary transmission and primary distribution and secondary distribution.

It may be noted that it is not necessary that all power schemes include all the stages.

(i)Generating station : In Fig, G.S. represents the generating station where electric power is produced by 3-phase alternators operating in parallel. The usual generation voltage is 11 kV. For economy in the transmission of electric power, the generation voltage (i.e., 11 kV) is stepped upto 132 kV (or more) at the generating station with the help of 3-phase transformers.

The transmission of electric power at high voltages has several advantages including the saving of conductor material and high transmission efficiency.

Generally the primary transmission is carried at 66 kV, 132 kV, 220 kV or 400 kV.

(ii) Primary transmission. 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.

(iii) 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.

(iv) 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.

(v) Secondary distribution. The electric power from primary distribution line (11 kV) is delivered to distribution sub-stations (DS). 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, whereas 3-phase, 400 V motor load is connected across 3-phase lines directly.