Typical A.C Power System Scheme

TYPICAL A.C POWER SYSTEM SCHEME

The large network of conductors between the power station and the consumers can be broadly divided into two parts viz., transmission system and distribution system. Each part can be further subdivided into two primary transmission, secondary transmission and primary distribution and secondary distribution. Fig. 7.1 shows the layout of a typical AC power supply scheme by a single line diagram.

1. Generating station

Generating station in which the electric power is produced by 3‒phase alternator operating in parallel. The usual generation voltage is 11 kV. The power generated at the generating stations is transmitted at higher voltage to the main load centres for economy purpose.

2. Primary transmission

Generally the primary transmission is carried at 66 kV, 132 kV, 220 kV or 400 kV. The electric power at 132 kV is transmitted by 3‒phase, 3‒wire overhead system. This forms the primary transmission. There are advantages when power is transmitted at high voltages:

(i) The volume of conductor material required is low.

(ii) For a given amount of power transmitted, the current through the line is reduced, when the transmission system voltage is high. This reduces the line losses and hence efficiency of transmission is reduced.

(iii) As the current is reduced the voltage in the line is reduced and hence the line regulation is improved.

3. Secondary transmission

The primary transmission line terminates at the receiving station. At the receiving station, the voltage is reduced to 33 kV by step down transformers. From this station, electric power is transmitted at 33 kV by 3‒phase, 3‒wire overhead system to various sub‒stations located at the strategic points in the city. This forms the secondary transmission.

4. Primary distribution

The secondary transmission line terminates at the sub‒station where the voltage is reduced from 33 kV to 11 kV, 3‒phase, 3‒wire system. The 11 kV lines run along the important road sides of the city. This forms the primary distribution.

5. Secondary distribution

The electric power from primary distribution line (11 kV) is delivered to distribution sub‒stations. These sub‒stations are located near the consumers localities and step down the voltage to 400 V, 3‒phase, 4‒wire system. The secondary distribution system consists of feeders, distributors and service mains. Fig. 7.2 shows the elements of low voltage distribution system. The distributors may be radial or ring. Feeders (SC or SA) radiating from the distribution substation (DS) supply power to the distributors (AB, BC, CD and DA). No consumers is given direct connection from the feeders. Instead, the consumers are connected to the distributors through their service mains.