Structure of power system
The electrical power produced in generating stations is transmitted Over large distances trough transmission lines. Then it is distributed to various consumers through a distribution network. Various stages are involved in between, in which this power is processed and sent further. A typical single line diagram of the A.C. power supply scheme is shown in Fig. 1. Standard transmission voltages are 132 kV, 220 kV, 400 kV.
This diagram gives a general idea of the supply system. The various stages are:
1) Generating station:
Power is generated at generating stations at 11 kV voltage level. (It may be 6.6 kV or 33 kV in some cases). It is stepped up to 132 kV for primary transmission purposes. A 3phase step-up transformer is used to step-up the voltage.
★ The advantages of transmitting power at high voltage are reduction in the size of the conductor, better performance of the transmission process.
★ The highest transmission voltage in India is 400 kV.
2) Primary Transmission: Power at high voltage i.e. 132 kV is transmitted by
means of primary transmission lines. It is 3-phase, 3-wire system (overhead tower
3) Secondary Transmission:
At the receiving station, the voltage is stepped down from 132 kV to 33 kV by means of a step-down transformer. From the receiving station, electric power is transmitted to a substation located at particular points in the city. The overhead 3-phase, 3-wire system (tower line or iron poles) is used for this.
4) Primary Distribution:
The secondary transmission line terminates at the receiving
station where the voltage is stepped down to 11 kV by step down transformer and these 11 kV lines run along roadsides of the city on steel poles.
The distribution substations are located at consumer’s premises where a voltage of 11 kV is stepped down to 400 volts (3-phase 4-wire) for secondary distribution. The advantages of using a 3-phase 4-wire system (R, Y, B, N) are that both 3 phase and 1 phase supply can be used depending upon the load. Service lines are taken off from this line
★ This one-line diagram can be represented by a simple block diagram as shown in
Fig.2. from generation to distribution of electrical energy.
★ The block diagram of energy conversion in the show in fig.3.