Page 1
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.
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, 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 sub-stations generally use synchronous condensers as the
power factor improvement equipment.
Page 2
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.
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, 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 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. 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 outdoor. 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. 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 complete
discussion on pole-mounted sub-station, the reader may refer to Art. 25.5.
Comparison between Outdoor and Indoor Sub-Stations:
The comparison between outdoor and indoor sub-stations is given below in the tabular form:
Page 3
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.
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, 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 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. 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 outdoor. 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. 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 complete
discussion on pole-mounted sub-station, the reader may refer to Art. 25.5.
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 the 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
indoor sub-station. In general, most of the sub-stations are of outdoor type and the indoor
sub-stations are erected only where outdoor construction is impracticable or prohibited by the local laws.
Transformer Sub-Stations
The majority of the sub-stations in the power system are concerned with the changing of voltage level of
electric supply. These are known as transformer sub-stations because 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. 25.1 shows the block diagram of a typical electric supply system indicating the position of 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.
Transfor mer Sub-Stations
Page 4
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.
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, 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 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. 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 outdoor. 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. 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 complete
discussion on pole-mounted sub-station, the reader may refer to Art. 25.5.
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 the 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
indoor sub-station. In general, most of the sub-stations are of outdoor type and the indoor
sub-stations are erected only where outdoor construction is impracticable or prohibited by the local laws.
Transformer Sub-Stations
The majority of the sub-stations in the power system are concerned with the changing of voltage level of
electric supply. These are known as transformer sub-stations because 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. 25.1 shows the block diagram of a typical electric supply system indicating the position of 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.
Transfor mer 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 economy in transmission of electric power. The sub-stations which accomplish this job are called
step-up sub-stations. These are generally located in the power houses and are of 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 secondary transmission. The primary grid
sub-station is generally of 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 road sides 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 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 consumer’s 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 majority of the distribution substations are of pole-mounted
type.
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
Page 5
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.
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, 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 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. 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 outdoor. 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. 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 complete
discussion on pole-mounted sub-station, the reader may refer to Art. 25.5.
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 the 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
indoor sub-station. In general, most of the sub-stations are of outdoor type and the indoor
sub-stations are erected only where outdoor construction is impracticable or prohibited by the local laws.
Transformer Sub-Stations
The majority of the sub-stations in the power system are concerned with the changing of voltage level of
electric supply. These are known as transformer sub-stations because 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. 25.1 shows the block diagram of a typical electric supply system indicating the position of 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.
Transfor mer 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 economy in transmission of electric power. The sub-stations which accomplish this job are called
step-up sub-stations. These are generally located in the power houses and are of 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 secondary transmission. The primary grid
sub-station is generally of 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 road sides 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 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 consumer’s 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 majority of the distribution substations are of pole-mounted
type.
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 25.2
(i) shows the layout of pole-mounted sub-station whereas Fig. 25.2
(ii) shows the schematic connections. The transformer and other equipment are mounted on 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 breaker (O.C.B.) installed
on the L.T. side automatically isolates the transformer from Sub-Station the consumers in the event of
any fault. The pole-mounted
sub-stations are generally used for transformer capacity upto *200 kVA. The following points may be
noted about pole-mounted sub-stations :
(i) There should be 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.
Underground Sub-Station:
In thickly populated cities, there is scarcity of land as well as the prices of land are very high. This has led
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