Distribution system  

Till now we know how power at somewhat high voltage (let say 33 kV) is received in a substation located near load center. The loads of a big city are mainly residential complexes, schools, offices, hotels, street lighting and so on. Such kinds of consumers are termed as LT (low tension) consumers. Apart from this there might be average and small scale industries situated in the outskirts of the city. LT consumers are to be supplied with single phase, 220 V, 40 Hz. The scheme is shown in figure below which illustrates how the above is attained in the substation receiving power at 33 kV.

                                              Figure: Typical Power distribution scheme.

Power receive at a 33 kV substation is primary stepped down to 6 kV and with the aid of underground cables (termed as feeder lines), power flow is directed to various directions of the city. At the final level, step down transformers are employed to step down the voltage ranging from 6 kV to 400 V. Such transformers are termed as distribution transformers with 400 V, star associated secondary. You should have observed such transformers mounted on poles in cities alongside the roads. These are termed as pole mounted substations. From the secondary of such transformers 4 terminals (R, Y, B & N) come out. ‘N’ is termed as the neutral and taken out from the common point of star connected secondary. The voltage among any two phases (that is, R-Y, Y-B and B-R) is 400 V and among any phase and neutral is 230 V (=400/√ 3). The residential buildings are supplied with single phase 230V, 50Hz. Therefore separate are to be supplied with any one of the phases and neutral. Supply authority tries to view that the loads stay evenly balanced between the phases as far as possible. That means approximately one third of the consumers will be supplied from R-N, next one third from Y-N and the remaining one third from B-N. The distribution of power from the pole mounted substation can be done either by (A) overhead lines (i.e., bare conductors) or by (B) underground cables. By using of overhead lines however cheap, is frequently accident prone and also theft of power by hooking from the lines occur. However costly, in big cities and thickly populated regions underground cables for distribution of power are employed.