The present-day electrical power system is a.c. i.e. electric power is generated, transmitted and distributed in the form of alternating current. The electric power is produced at the power stations which are located at favorable places, generally quite away from the consumers. It is delivered to the consumers through a large network of transmission and distribution. At many places in the line of the power system, it may be desirable and necessary to change some characteristic (e.g. voltage, d.c., frequency, p.f. etc.) of electric supply. This is accomplished by suitable apparatus called sub-station. For example, generation voltage (11 kV or 6·6 kV) at the power station is stepped up to high voltage (say 220 kV or 132 kV) for transmission of electric power. The assembly of apparatus (e.g. transformer etc.) used for this purpose is the sub-station. Similarly, near the consumer’s localities, the voltage may have to be stepped down to utilization level. This job is again

accomplished by a suitable apparatus called sub-station. Yet at some places in the line of the power system, it may be desirable to convert large quantities of a.c. power to d.c. power e.g. for traction, electroplating, *d.c. motors etc. This job is again performed by suitable apparatus (e.g. Ignitron) called sub-station. It is clear that type of equipment needed in a sub-station will depend upon the service requirement. Although there can be several types of sub-stations, we shall mainly confine our attention to only those sub-stations where the incoming and outgoing supplies are a.c. i.e. sub-stations which change the voltage level of the electric supply.

The assembly of apparatus used to change some characteristic (e.g. voltage, a.c. to d.c., frequency, p.f. etc.) of electric supply is called a sub-station.

Sub-stations are important part of power system. The continuity of supply depends to a considerable extent upon the successful operation of sub-stations. It is, therefore, essential to exercise utmost care while designing and building a sub-station. The following are the important points which must be kept in view while laying out a sub-station:

(i) It should be located at a proper site. As far as possible, it should be located at the centre of gravity of load.
(ii) It should provide safe and reliable arrangement. For safety, consideration must be given to the maintenance of regulation clearances, facilities for carrying out repairs and maintenance, abnormal occurrences such as possibility of explosion or fire etc. For reliability, consideration must be given for good design and construction, the provision of suitable protective gear etc.
(iii) It should be easily operated and maintained.
(iv) It should involve minimum capital cost.

“Power system by” V.K MEHTA.

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