SUBSTATION & THEIR CLASSIFICATIONS
Sub-station
The assembly of apparatus used to change some characteristics (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 the outmost 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 center 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 the 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 a 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, 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 o! 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 utilization.
(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. ignition) 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 indoor because of economic considerations. However, when the atmosphere is contaminated with impurities, these sub-stations can be erected for voltages up to 66 kV.
(ii) Outdoor sub-stations:- For voltages beyond 66 kV, equipment is invariably installed outdoor. It is because, for such voltages, the clearances between conductors and the space required for switches, circuit breakers and other equipment become 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. The reader may find further discussion on underground sub-stations in Art. 25.6.
(iv) Pole-mounted sub-stations. This is an outdoor sub-station 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. For a complete discussion on pole-mounted sub-station.
Comparison between Outdoor and Indoor Sub-Stations
The comparison between outdoor and indoor sub-stations is given below in the tabular form :
From the above comparison, it is clear that each type has its own advantages and disadvantages. However, comparative economics (i.e. annual cost of operation) is tile most powerful factor influencing the choice between indoor and outdoor sub-stations. The greater cost of indoor sub-station prohibits its use. But sometimes non-economic factors (e.g. public safety) exert considerable influence in choosing an indoor sub-station. In general, most of the sub-stations are of an outdoor type and the indoor sub-stations are erected only where outdoor construction is impracticable or prohibited by the local la
Transformer Sub-Stations
The majority of the sub-stations in the power system are concerned with the changing of the voltage level of electric supply. These are known as transformer sub-stations because the transformer is the main component employed to change the voltage level. Depending upon the purpose served, transformer sub-stations may be classified into :
(i) Step-up sub-station (ii) Primary grid sub-station
(iii) Secondary sub-station (iv) Distribution sub-station
Fig. shows the block diagram of a typical electric supply system indicating the position of the above types of sub-stations. It may be noted that it is not necessary that all electric supply schemes
include all the stages shown in the figure. For example,. in a certain supply scheme there may not be secondary sub-stations and in another case, the scheme may be so small that there are only distribution sub-stations.
(i) Step-up sub-station:- The generation voltage (11 kV in this case) is stepped up to high voltage (220 kV) to affect the economy in the transmission of electric power. The sub-stations which accomplish this job are called step-up sub-stations. These are generally located in the powerhouses and are of the outdoor type.
(ii) Primary grid sub-station:- From the step-up sub-station, electric power at 220 kV is transmitted by 3-phase, 3-wire overhead system to the outskirts of the city. Here, electric power is received by the primary grid sub-station which reduces the voltage level to 66 kV for the secondary transmission. The primary grid sub-station is generally of the outdoor type.
(iii) Secondary sub-station:- From the primary grid sub-station, electric power is transmitted at 66 kV by 3-phase, 3-wire system to various secondary sub-stations located at the strategic points in the city. At a secondary sub-station, the voltage is further stepped down to 11 kV. The 11 kV lines run along the important roadsides of the city. It may be noted that big consumers (having demand more than 50 kW) are generally supplied power at 11 kV for further handling with their own sub-stations. The secondary sub-stations are also generally of the outdoor type.
(iv) Distribution sub-station:- The electric power from 11 kV lines is delivered to distribution sub-stations. These sub-stations are located near the consumers' localities and step down the voltage to 400 V, 3-phase, 4-wire for supplying to the consumers. The voltage between any two phases is 400V and between any phase and neutral, it is 230 V. The single-phase residential lighting load is connected between any one phase and neutral whereas 3-phase, 400V motor load is connected across 3-phase lines directly. It may be worthwhile to mention here that the majority of the distribution substations are of pole-mounted.
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. (i) shows the layout of pole-mounted sub-station
(ii) 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 (11kV / 400 V) through gang isolator and fuses. The lightning arresters are installed on the H.T. side to protect the sub-station 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 (O.C.B.) installed on the L.T. sub-stations 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 isolators and O.C.B. should be shut off.
Underground Sub-Station
In thickly populated cities, there is a scarcity of land, as well as the prices of land, are very high. This has led to the development of an underground substation. In such sub-stations, the equipment is placed underground.
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