Question

For transmitting same amount 1 point
of power over the same
distance, the amount of
conductor material required is
O
More in 34 system
O
Both b&c.
O
Less in 34 system
Less in 2 system​

Answer 1

Answer:

1604 Electrical Technology

41.1. General Layout of the System

The conductor system by means of which electric power is conveyed from a generating station to

the consumer’s premises may, in general, be divided into two distinct parts i.e. transmission system

and distribution system. Each part can again be sub-divided into two—primary transmission and

secondary transmission and similarly, primary distribution and secondary distribution and then

finally the system of supply to individual consumers. A typical layout of a generating, transmission

and distribution network of a large system would be made up of elements as shown by a single-line

diagram of Fig. 41.1 although it has to be realized that one or more of these elements may be missing

in any particular system. 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.

Now-a-days, generation and transmission is almost exclusively three-phase. The secondary trans-

mission 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.

In Fig. 41.1, C.S. represents the central station where power is generated by 3-phase alternators

at 6.6 or 11 or 13.2 or even 32 kV. 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. Primary or high-voltage transmission is carried out at 132 kV*. 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. Hence, the economical

voltage of transmission is that for which the saving in copper or aluminium is not offset (i) by the

increased cost of insulating the line (ii) by the increased size of transmission-line structures and

(iii) by the increased size of generating stations and sub-stations. A rough basis of determining the

most economical transmission voltage is to use 650 volt per km of transmission line. For example, if

transmission line is 200 km, then the most economical transmission voltage will be 200 × 650

≅ 132,000 V or 132 kV.

The 3-phase, 3-wire overhead high-voltage transmission line next terminates in step-down trans-

formers in a sub-station known as Receiving Station (R.S.) which usually lies at the outskirts of a city

because it is not safe to bring high-voltage overhead transmission lines into thickly-populated areas.

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.

From the Receiving Station, power is next transmitted at 33 kV by underground cables (and

occasionally by overhead lines) to various sub-stations (SS) located at various strategic points in the

city. This is known as secondary or low-voltage transmission. From now onwards starts the primary

and secondary distribution.

At the sub-station (SS) voltage is reduced from 33kV to 3.3kV 3-wire for primary distribution.

Consumers whose demands exceeds 50 kVA are usually supplied from SS by special 3.3 kV feeders.

The secondary distribution is done at 400/230 V for which purpose voltage is reduced from

3.3 kV to 400 V at the distribution sub-stations. Feeders radiating from distribution sub-station

supply power to distribution networks in their respective areas. If the distribution network happens to

be at a great distance from sub-station, 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 distribu-

tion networks. The most common system for secondary distribution is 400/230-V, 3-phase 4-wire

system. The single-phase residential lighting load is connected between any one line and the neutral

* High voltages like 750 kV are in use in USSR (Konakovo-Moscow line) and 735 kV in Canada (Montreal-

Manicoagan Scheme).