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 favourable 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, a.c. to d.c., frequency, p.f. etc.) of electric supply. This is accomplished by a suitable apparatus called a 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 consumers localities, the voltage may have to be stepped down to utilisation 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 the 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.
Sub-Station
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 an important part of the 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 the load.
(ii) It should provide a 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.
Classification of Sub-Stations
There are several ways of classifying sub-stations. However, the two most important ways of classifying them are according to (1) service requirement and (2) constructional features.
1. According to service requirement. A sub-station may be called upon to change voltage level or improve power factor or convert a.c. power into d.c. power etc. According to the service requirement, sub-stations may be classified into :
(i) Transformer sub-stations. Those sub-stations which change the voltage level of electric supply are called transformer sub-stations. These sub-stations receive power at some voltage and deliver it at some other voltage. Obviously, transformers will be the main component in such sub-stations. Most of the sub-stations in the power system are of this type.
(ii) Switching sub-stations. These sub-stations do not change the voltage level i.e. incoming and outgoing lines have the same voltage. However, they simply perform the switching operations of power lines.
(iii) Power factor correction sub-stations. Those sub-stations which improve the power factor of the system are called power factor correction sub-stations. Such sub-stations are generally located at the receiving end of transmission lines. These sub-stations generally use synchronous condensers as the power factor improvement equipment.
(iv) Frequency changer sub-stations. Those sub-stations which change the supply frequency are known as frequency changer sub-stations. Such a frequency change may be required for industrial utilisation.
(v) Converting sub-stations. Those sub-stations which change a.c. power into d.c. power are called converting sub-stations. These sub-stations receive a.c. power and convert it into d.c. power with suitable apparatus (e.g. ignitron) to supply for such purposes as traction, electroplating, electric welding etc.
(vi) Industrial sub-stations. Those sub-stations which supply power to individual industrial concerns are known as industrial sub-stations.
2. According to constructional features. A sub-station has many components (e.g. circuit breakers, switches, fuses, instruments etc.) which must be housed properly to ensure continuous and reliable service. According to constructional features, the sub-stations are classified as :
(i) Indoor sub-station
(ii) Outdoor sub-station
(iii) Underground sub-station
(iv) Pole-mounted sub-station
(i) Indoor sub-stations. For voltages upto 11 kV, the equipment of the sub-station is installed indoor because of economic considerations. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltages upto 66 kV.
(ii) Outdoor sub-stations. For voltages beyond 66 kV, equipment is invariably installed out door. It is because for such voltages, the clearances between conductors and the space required for switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoor.
(iii) Underground sub-stations. In thickly populated areas, the space available for equipment and building is limited and the cost of land is high. Under such situations, the sub-station is created underground.
(iv) Pole-mounted sub-stations. This is an outdoor sub-station with equipment installed over head on H-pole or 4-pole structure. It is the cheapest form of sub-station for voltages not exceeding 11kV (or 33 kV in some cases). Electric power is almost distributed in localities through such sub stations.
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