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Introduction


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 produce at the power station, which are located at favorable places, generally quite away from the consumers. It is delivered to the consumer through a large network of transmission and distribution. At many place in the line of power system, it may be desirable and necessary to change some characteristic ( e.g. Voltage, ac to de, frequency p.f. etc.) of electric supply. This is accomplished by suitable apparatus called sub-station for example, generation voltage (11kv or 6.6kv) at the power station is stepped up to high voltage (Say 220kv to 132kv) for transmission of electric power. Similarly near the consumer’s localities, the voltage may have to be stepped down to utilization level. This job is again accomplished by suitable apparatus called sub-station.

Definition of sub-station :-
“The assembly of apparatus used to change some characteristics (e.g. Voltage al to de freq. p.f. etc) of electric supply is called sub-station”





Classification of sub-station
There are several ways of classifying sub-station. However the two most important way of classifying them are.
I) According to service requirement :-
According to service requirement sub-station may be classified into.
1) Transformer sub-station :-
Those sub-station which change the voltage level of electrical supply are called TIF s/s.
2) Switching sub-station :-
These sub-station simply perform the switching operation of power line.
3) Power factor correction S/S :-
These sub-station which improve the p.f. of the system are called p.f. correction s/s. these are generally located at receiving end s/s.
4) Frequency changer S/S :-
Those sub-stations, which change the supply frequency, are known as frequency changer s/s. Such s/s may be required for industrial utilization.
5) Converting sub-station :-
Those sub-station which change a.c. power into d.c. power are called converting s/s ignition is used to convert AC to dc power for traction, electroplating, electrical welding etc.


6) Industrial sub-station :-
Those sub-stations, which supply power to individual industrial concerns, are known as industrial sub-station.