The present-day electrical power system generally generates, is transmitted and distributed in form of alternating current. The electric power is produced at the power stations which are located in far places, generally 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 characteristics (e.g. voltage, A.C. to D.C, frequency, p.f. etc.) of the electric supply. This is acquired by suitable apparatus called a substation. For example, the 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.
Therefore substation is defined as the assembly of apparatus where we change the characteristics of power such as voltage, frequency, power factor, waveform(AC-DC or DC-AC) etc.
It is clear that type of equipment needed in a sub-station will depend upon the service requirement.
Classification of Sub-Station. There are several ways of classifying sub-stations. However, the two most important ways of classifying them are according to
(1) service requirements and
(2) constructional features.
1. According to the 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 the electric supply are called transformer sub-stations. These sub-stations receive power at some voltage and deliver it at some other voltage. Obviously, the transformer will be the main component in such substations. 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 substations 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 utilization.
(v) Converting sub-station.
Those sub-stations which change a.c. power into d.c. power is called converting sub-stations. These sub-stations receive a.c. power and convert it into d.c. power with suitable apparatus (e.g. inverter) 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, up to 11 kV, the equipment of the sub-station is installed indoors because of economic considerations. However, when the atmosphere is contaminated with impurities, these substations can be erected for voltages up to 66 kV.
(ii) Outdoor sub-stations.
For voltages beyond 66 kV, equipment is invariably installed outdoors. 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 indoors.
(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 substation with equipment installed overhead 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 substations.
Comparison between Outdoor and Indoor Sub-Stations
The comparison between outdoor and indoor sub-stations is given below in tabular form:
Pole-Mounted Sub-Station:
It is a distribution sub-station placed overhead on a pole. It is the cheapest form of sub-station as it does not involve any building work. Fig shows the layout of the pole-mounted substation whereas Fig. 2nd shows the schematic connections. The transformer and other equipment are mounted on an H-type pole (or 4-pole structure). The 11 kV line is connected to the transformer (11 kV / 400 V) through gang isolator and fuses. The lightning arresters are installed on the H.T. side to protect the substation from lightning strokes. The transformer steps down the voltage to 400V, 3-phase, 4-wire supply. The voltage between any two lines is 400V whereas the voltage between any line and neutral is 230 V. The oil circuit breaker (O.C.B.) installed on the L.T. side automatically isolates the transformer from the consumers in the event of any fault. The pole-mounted substations are generally used for transformer capacity up to 200 kVA.
The following points may be noted about pole-mounted sub-stations :
(i) There should be a periodical check-up of the dielectric strength of oil in the transformer and O.C.B. (ii) In case of repair of transformer or O.C.B., both gang isolator and O.C.B. should be shut off.
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| 11KV/440V pole mounted substation @ GNI Nagpur |
