Power Generation: Substations | Power Systems - Electrical Engineering (EE) PDF Download

 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