For example, in a certain system, there may be no secondary transmission and in another case, when the generating station is nearby, there may be no transmission and the distribution system proper may begin at the generator bus-bars. The secondary single phase alternator pdf is also 3-phase whereas the distribution to the ultimate customer may be 3-phase or single- phase depending upon the requirements of the customers.
Single-line diagram of transmission and distribution network. The voltage is then stepped up by suitable 3-phase transformers for transmission purposes. Taking the generated voltage as 11 kV, the 3-phase transformers step it up to 132 kV as shown. The transmission voltage is, to a very large extent, determined by economic considerations. High voltage transmission requires conductors of smaller cross-section which results in economy of copper or aluminium. But at the same time cost of insulating the line and other expenses are increased. The increased size of generating stations and substations.
A rough basis of determining the most economical transmission voltage is to use 650 volt per km of transmission line. Here, the voltage is stepped down to 33 kV. It may be noted here that for ensuring continuity of service transmission is always by duplicate lines. This is known as secondary or low-voltage transmission.
From now onwards starts the primary and secondary distribution. Consumers whose demands exceeds 50 kVA are usually supplied from SS by special 3. 230 V for which purpose voltage is reduced from 3. 3 kV to 400 V at the distribution substations. Feeders radiating from distribution substation supply power to distribution networks in their respective areas.
If the distribution network happens to be at a great distance from substation, then they are supplied from the secondaries of distribution transformers which are either pole-mounted or else housed in kiosks at suitable points of the distribution networks. The single-phase residential lighting load is connected between any one line and the neutral whereas 3-phase, 400-V motor load is connected across 3-phase lines directly. It should be noted that low-voltage distribution system is sub-divided into feeders, distributors and service mains. No consumer is given direct connection from the feeders, instead consumers are connected to distribution network through their service mains. High voltage transmission: 220 kV, 132 kV, 66kV.