Detailed Notes: Distribution System | Power Systems - Electrical Engineering (EE) PDF Download

 Page 1


  
 
  
DISTRIBUTION SYSTEMS 
 
5.1 DISTRIBUTION SYSTEM:  
That part of power system which distributes electric power for local use is known as distribution system.  
In general, the distribution system is the electrical system between the sub-station fed by the distribution 
system and the consumer‘s meters. It generally consists of feeders, distributors and the service mains.  
(i) Feeders: A feeder is a conductor which connects the sub-station (or localized generating station) to the 
area where power is to be distributed. Generally, no tapping‘s are taken from the feeder so that current in it 
remains the same throughout. The main consideration in the design of a feeder is the current carrying 
capacity.  
 
(ii) Distributor. A distributor is a conductor from which tappings are taken for supply to the consumers. In 
Fig. 12.1, AB, BC, CD and DA are the distributors. The current through a distributor is not constant because 
tappings are taken at various places along its length. While designing a distributor, voltage drop along its 
length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the 
consumers‘ terminals.  
i. Service mains. A service mains is generally a small cable which connects the distributor to the 
consumers‘ terminals. 
Classification of Distribution Systems  
A distribution system may be classified according to ;  
(i) Nature of current. According to nature of current, distribution system may be classified as (a) 
d.c. distribution system (b) a.c. distribution system.  
Now-a-days, a.c. system is universally adopted for distribution of electric power as it is simpler and 
more economical than direct current method.  
ii. Type of construction. According to type of construction, distribution system may be classified as 
(a)       overhead system (b) underground system. The overhead system is generally employed for 
distribution as it is 5 to 10 times cheaper than the equivalent underground system. In general, the 
Page 2


  
 
  
DISTRIBUTION SYSTEMS 
 
5.1 DISTRIBUTION SYSTEM:  
That part of power system which distributes electric power for local use is known as distribution system.  
In general, the distribution system is the electrical system between the sub-station fed by the distribution 
system and the consumer‘s meters. It generally consists of feeders, distributors and the service mains.  
(i) Feeders: A feeder is a conductor which connects the sub-station (or localized generating station) to the 
area where power is to be distributed. Generally, no tapping‘s are taken from the feeder so that current in it 
remains the same throughout. The main consideration in the design of a feeder is the current carrying 
capacity.  
 
(ii) Distributor. A distributor is a conductor from which tappings are taken for supply to the consumers. In 
Fig. 12.1, AB, BC, CD and DA are the distributors. The current through a distributor is not constant because 
tappings are taken at various places along its length. While designing a distributor, voltage drop along its 
length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the 
consumers‘ terminals.  
i. Service mains. A service mains is generally a small cable which connects the distributor to the 
consumers‘ terminals. 
Classification of Distribution Systems  
A distribution system may be classified according to ;  
(i) Nature of current. According to nature of current, distribution system may be classified as (a) 
d.c. distribution system (b) a.c. distribution system.  
Now-a-days, a.c. system is universally adopted for distribution of electric power as it is simpler and 
more economical than direct current method.  
ii. Type of construction. According to type of construction, distribution system may be classified as 
(a)       overhead system (b) underground system. The overhead system is generally employed for 
distribution as it is 5 to 10 times cheaper than the equivalent underground system. In general, the 
  
 
underground system is used at places where overhead construction is impracticable or prohibited 
by the local laws.  
iii. Scheme of connection. According to scheme of connection, the distribution system may be 
classified as (a) radial system (b) ring main system (c) inter-connected system. Each scheme has its 
own advantages and disadvantages 
5.2 A.C. DISTRIBUTION  
i. Now-a-days electrical energy is generated, transmitted and distributed in the form of alternating 
current.  
ii. Alternating current in preferred to direct current is the fact that alternating voltage can be 
conveniently changed by means of a transformer.  
iii. High distribution and distribution voltages have greatly reduced the current in the conductors and 
the resulting line losses.  
iv. The A.C. distribution system is the electrical system between the step-down substation fed by the 
distribution system and the consumers‘ meters.  
The A.C. distribution system is classified into  
(i) Primary distribution system and  
(ii) Secondary distribution system. 
5.2.1 PRIMARY DISTRIBUTION SYSTEM: 
 
 
It is that part of A.C. distribution system which operates at voltages somewhat higher than general 
utilization than the average low-voltage consumer uses.  
 The most commonly used primary distribution voltages are 11 kV, 6·6kV and 3·3 kV  
 
Primary distribution is carried out by 3-phase, 3-wire system.   Fig. shows a typical primary distribution 
system.  Electric power from the generating station is transmitted at high voltage to the substation located 
Page 3


  
 
  
DISTRIBUTION SYSTEMS 
 
5.1 DISTRIBUTION SYSTEM:  
That part of power system which distributes electric power for local use is known as distribution system.  
In general, the distribution system is the electrical system between the sub-station fed by the distribution 
system and the consumer‘s meters. It generally consists of feeders, distributors and the service mains.  
(i) Feeders: A feeder is a conductor which connects the sub-station (or localized generating station) to the 
area where power is to be distributed. Generally, no tapping‘s are taken from the feeder so that current in it 
remains the same throughout. The main consideration in the design of a feeder is the current carrying 
capacity.  
 
(ii) Distributor. A distributor is a conductor from which tappings are taken for supply to the consumers. In 
Fig. 12.1, AB, BC, CD and DA are the distributors. The current through a distributor is not constant because 
tappings are taken at various places along its length. While designing a distributor, voltage drop along its 
length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the 
consumers‘ terminals.  
i. Service mains. A service mains is generally a small cable which connects the distributor to the 
consumers‘ terminals. 
Classification of Distribution Systems  
A distribution system may be classified according to ;  
(i) Nature of current. According to nature of current, distribution system may be classified as (a) 
d.c. distribution system (b) a.c. distribution system.  
Now-a-days, a.c. system is universally adopted for distribution of electric power as it is simpler and 
more economical than direct current method.  
ii. Type of construction. According to type of construction, distribution system may be classified as 
(a)       overhead system (b) underground system. The overhead system is generally employed for 
distribution as it is 5 to 10 times cheaper than the equivalent underground system. In general, the 
  
 
underground system is used at places where overhead construction is impracticable or prohibited 
by the local laws.  
iii. Scheme of connection. According to scheme of connection, the distribution system may be 
classified as (a) radial system (b) ring main system (c) inter-connected system. Each scheme has its 
own advantages and disadvantages 
5.2 A.C. DISTRIBUTION  
i. Now-a-days electrical energy is generated, transmitted and distributed in the form of alternating 
current.  
ii. Alternating current in preferred to direct current is the fact that alternating voltage can be 
conveniently changed by means of a transformer.  
iii. High distribution and distribution voltages have greatly reduced the current in the conductors and 
the resulting line losses.  
iv. The A.C. distribution system is the electrical system between the step-down substation fed by the 
distribution system and the consumers‘ meters.  
The A.C. distribution system is classified into  
(i) Primary distribution system and  
(ii) Secondary distribution system. 
5.2.1 PRIMARY DISTRIBUTION SYSTEM: 
 
 
It is that part of A.C. distribution system which operates at voltages somewhat higher than general 
utilization than the average low-voltage consumer uses.  
 The most commonly used primary distribution voltages are 11 kV, 6·6kV and 3·3 kV  
 
Primary distribution is carried out by 3-phase, 3-wire system.   Fig. shows a typical primary distribution 
system.  Electric power from the generating station is transmitted at high voltage to the substation located 
  
 
in or near the city. At this substation, voltage is stepped down to 11 kV with the help of step-down 
transformer. Power is supplied to various substations for distribution or to big consumers at this voltage. 
This forms the high voltage distribution or primary distribution.  
5.2.2 SECONDARY DISTRIBUTION SYSTEM: 
 
 
It is that part of a.c. distribution system employs 400/230 V, 3-phase, 4-wire system.  
shows a typical secondary distribution system.  
The primary distribution circuit delivers power to various substations, called distribution substations. The 
substations are situated near the consumers‘ localities and contain step down transformers.  
At each distribution substation, the voltage is stepped down to 400 V and power is delivered by 3-phase,4-
wire a.c. system.  
The voltage between any two phases is 400 V and between any phase and neutral is 230 V.  
The single phase domestic loads are connected between any one phase and the neutral, Motor loads are 
connected across 3-phase lines directly.  
5.3 D.C. DISTRIBUTION:  
For certain applications, d.c. supply is absolutely necessary. d.c. supply is required for the operation of 
variable speed machinery (i.e., d.c. motors storage battery.  
For this purpose, A.C. power is converted into D.C. power at the substation by using converting machinery 
e.g., mercury arc rectifiers, rotary converters and motor-generator sets.  
Page 4


  
 
  
DISTRIBUTION SYSTEMS 
 
5.1 DISTRIBUTION SYSTEM:  
That part of power system which distributes electric power for local use is known as distribution system.  
In general, the distribution system is the electrical system between the sub-station fed by the distribution 
system and the consumer‘s meters. It generally consists of feeders, distributors and the service mains.  
(i) Feeders: A feeder is a conductor which connects the sub-station (or localized generating station) to the 
area where power is to be distributed. Generally, no tapping‘s are taken from the feeder so that current in it 
remains the same throughout. The main consideration in the design of a feeder is the current carrying 
capacity.  
 
(ii) Distributor. A distributor is a conductor from which tappings are taken for supply to the consumers. In 
Fig. 12.1, AB, BC, CD and DA are the distributors. The current through a distributor is not constant because 
tappings are taken at various places along its length. While designing a distributor, voltage drop along its 
length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the 
consumers‘ terminals.  
i. Service mains. A service mains is generally a small cable which connects the distributor to the 
consumers‘ terminals. 
Classification of Distribution Systems  
A distribution system may be classified according to ;  
(i) Nature of current. According to nature of current, distribution system may be classified as (a) 
d.c. distribution system (b) a.c. distribution system.  
Now-a-days, a.c. system is universally adopted for distribution of electric power as it is simpler and 
more economical than direct current method.  
ii. Type of construction. According to type of construction, distribution system may be classified as 
(a)       overhead system (b) underground system. The overhead system is generally employed for 
distribution as it is 5 to 10 times cheaper than the equivalent underground system. In general, the 
  
 
underground system is used at places where overhead construction is impracticable or prohibited 
by the local laws.  
iii. Scheme of connection. According to scheme of connection, the distribution system may be 
classified as (a) radial system (b) ring main system (c) inter-connected system. Each scheme has its 
own advantages and disadvantages 
5.2 A.C. DISTRIBUTION  
i. Now-a-days electrical energy is generated, transmitted and distributed in the form of alternating 
current.  
ii. Alternating current in preferred to direct current is the fact that alternating voltage can be 
conveniently changed by means of a transformer.  
iii. High distribution and distribution voltages have greatly reduced the current in the conductors and 
the resulting line losses.  
iv. The A.C. distribution system is the electrical system between the step-down substation fed by the 
distribution system and the consumers‘ meters.  
The A.C. distribution system is classified into  
(i) Primary distribution system and  
(ii) Secondary distribution system. 
5.2.1 PRIMARY DISTRIBUTION SYSTEM: 
 
 
It is that part of A.C. distribution system which operates at voltages somewhat higher than general 
utilization than the average low-voltage consumer uses.  
 The most commonly used primary distribution voltages are 11 kV, 6·6kV and 3·3 kV  
 
Primary distribution is carried out by 3-phase, 3-wire system.   Fig. shows a typical primary distribution 
system.  Electric power from the generating station is transmitted at high voltage to the substation located 
  
 
in or near the city. At this substation, voltage is stepped down to 11 kV with the help of step-down 
transformer. Power is supplied to various substations for distribution or to big consumers at this voltage. 
This forms the high voltage distribution or primary distribution.  
5.2.2 SECONDARY DISTRIBUTION SYSTEM: 
 
 
It is that part of a.c. distribution system employs 400/230 V, 3-phase, 4-wire system.  
shows a typical secondary distribution system.  
The primary distribution circuit delivers power to various substations, called distribution substations. The 
substations are situated near the consumers‘ localities and contain step down transformers.  
At each distribution substation, the voltage is stepped down to 400 V and power is delivered by 3-phase,4-
wire a.c. system.  
The voltage between any two phases is 400 V and between any phase and neutral is 230 V.  
The single phase domestic loads are connected between any one phase and the neutral, Motor loads are 
connected across 3-phase lines directly.  
5.3 D.C. DISTRIBUTION:  
For certain applications, d.c. supply is absolutely necessary. d.c. supply is required for the operation of 
variable speed machinery (i.e., d.c. motors storage battery.  
For this purpose, A.C. power is converted into D.C. power at the substation by using converting machinery 
e.g., mercury arc rectifiers, rotary converters and motor-generator sets.  
  
 
The D.C. supply obtained in the form of (i) 2-wire or (ii) 3-wire for distribution.  
i. 2-Wire D.C. System  
? As the name implies, this system of distribution consists of two wires.  
? One is the outgoing or positive wire and the other is the return or negative wire.  
? The loads such as lamps, motors etc. are connected in parallel between the two wires as shown 
in Fig.  
 
This system is never used for distrubution purposes due to low efficiency but may be employed for 
distribution of d.c. power.  
 
ii. 3-wire D.C. system.  
? It consists of two outers and a middle or neutral wire which is earthed at the substation.  
? The voltage between the outers is twice the voltage between either outer or neutral.  
? The principal advantage of this system is that it makes available two voltages at the consumer 
terminals,  
? V between any outer and the neutral and 2V between the outers.  
5.4 COMPARISON OF D.C. AND A.C. DISTRIBUTION:  
The electric power can be distributed either by means of D.C. or A.C. Each system has its own merits and 
demerits. 
5.4.1 D.C DISTRIBUTION:  
? Advantages.  
(1). It requires only two conductors as compared to three for A.C. distribution.  
(2). There is no inductance, capacitance, phase displacement and surge problems in D.C. 
distribution.  
(3). Due to the absence of inductance, the voltage drop in a D.C. distribution line is less than the 
A.C. line for the same load and sending end voltage. For this reason, a D.C. distribution line 
has better voltage regulation.  
(4). There is no skin effect in a D.C. system. Therefore, entire cross-section of the line conductor is 
utilized.  
(5). For the same working voltage, the potential stress on the insulation is less in case of D.C. 
system than that in A.C. system. Therefore, a D.C. line requires less insulation.  
(6). A D.C. line has less corona loss and reduced interference with communication circuits.  
(7). The high voltage D.C. distribution is free from the dielectric losses, particularly in (viii) In D.C. 
distribution; there are no stability problems and synchronizing difficulties.  
 
Page 5


  
 
  
DISTRIBUTION SYSTEMS 
 
5.1 DISTRIBUTION SYSTEM:  
That part of power system which distributes electric power for local use is known as distribution system.  
In general, the distribution system is the electrical system between the sub-station fed by the distribution 
system and the consumer‘s meters. It generally consists of feeders, distributors and the service mains.  
(i) Feeders: A feeder is a conductor which connects the sub-station (or localized generating station) to the 
area where power is to be distributed. Generally, no tapping‘s are taken from the feeder so that current in it 
remains the same throughout. The main consideration in the design of a feeder is the current carrying 
capacity.  
 
(ii) Distributor. A distributor is a conductor from which tappings are taken for supply to the consumers. In 
Fig. 12.1, AB, BC, CD and DA are the distributors. The current through a distributor is not constant because 
tappings are taken at various places along its length. While designing a distributor, voltage drop along its 
length is the main consideration since the statutory limit of voltage variations is ± 6% of rated value at the 
consumers‘ terminals.  
i. Service mains. A service mains is generally a small cable which connects the distributor to the 
consumers‘ terminals. 
Classification of Distribution Systems  
A distribution system may be classified according to ;  
(i) Nature of current. According to nature of current, distribution system may be classified as (a) 
d.c. distribution system (b) a.c. distribution system.  
Now-a-days, a.c. system is universally adopted for distribution of electric power as it is simpler and 
more economical than direct current method.  
ii. Type of construction. According to type of construction, distribution system may be classified as 
(a)       overhead system (b) underground system. The overhead system is generally employed for 
distribution as it is 5 to 10 times cheaper than the equivalent underground system. In general, the 
  
 
underground system is used at places where overhead construction is impracticable or prohibited 
by the local laws.  
iii. Scheme of connection. According to scheme of connection, the distribution system may be 
classified as (a) radial system (b) ring main system (c) inter-connected system. Each scheme has its 
own advantages and disadvantages 
5.2 A.C. DISTRIBUTION  
i. Now-a-days electrical energy is generated, transmitted and distributed in the form of alternating 
current.  
ii. Alternating current in preferred to direct current is the fact that alternating voltage can be 
conveniently changed by means of a transformer.  
iii. High distribution and distribution voltages have greatly reduced the current in the conductors and 
the resulting line losses.  
iv. The A.C. distribution system is the electrical system between the step-down substation fed by the 
distribution system and the consumers‘ meters.  
The A.C. distribution system is classified into  
(i) Primary distribution system and  
(ii) Secondary distribution system. 
5.2.1 PRIMARY DISTRIBUTION SYSTEM: 
 
 
It is that part of A.C. distribution system which operates at voltages somewhat higher than general 
utilization than the average low-voltage consumer uses.  
 The most commonly used primary distribution voltages are 11 kV, 6·6kV and 3·3 kV  
 
Primary distribution is carried out by 3-phase, 3-wire system.   Fig. shows a typical primary distribution 
system.  Electric power from the generating station is transmitted at high voltage to the substation located 
  
 
in or near the city. At this substation, voltage is stepped down to 11 kV with the help of step-down 
transformer. Power is supplied to various substations for distribution or to big consumers at this voltage. 
This forms the high voltage distribution or primary distribution.  
5.2.2 SECONDARY DISTRIBUTION SYSTEM: 
 
 
It is that part of a.c. distribution system employs 400/230 V, 3-phase, 4-wire system.  
shows a typical secondary distribution system.  
The primary distribution circuit delivers power to various substations, called distribution substations. The 
substations are situated near the consumers‘ localities and contain step down transformers.  
At each distribution substation, the voltage is stepped down to 400 V and power is delivered by 3-phase,4-
wire a.c. system.  
The voltage between any two phases is 400 V and between any phase and neutral is 230 V.  
The single phase domestic loads are connected between any one phase and the neutral, Motor loads are 
connected across 3-phase lines directly.  
5.3 D.C. DISTRIBUTION:  
For certain applications, d.c. supply is absolutely necessary. d.c. supply is required for the operation of 
variable speed machinery (i.e., d.c. motors storage battery.  
For this purpose, A.C. power is converted into D.C. power at the substation by using converting machinery 
e.g., mercury arc rectifiers, rotary converters and motor-generator sets.  
  
 
The D.C. supply obtained in the form of (i) 2-wire or (ii) 3-wire for distribution.  
i. 2-Wire D.C. System  
? As the name implies, this system of distribution consists of two wires.  
? One is the outgoing or positive wire and the other is the return or negative wire.  
? The loads such as lamps, motors etc. are connected in parallel between the two wires as shown 
in Fig.  
 
This system is never used for distrubution purposes due to low efficiency but may be employed for 
distribution of d.c. power.  
 
ii. 3-wire D.C. system.  
? It consists of two outers and a middle or neutral wire which is earthed at the substation.  
? The voltage between the outers is twice the voltage between either outer or neutral.  
? The principal advantage of this system is that it makes available two voltages at the consumer 
terminals,  
? V between any outer and the neutral and 2V between the outers.  
5.4 COMPARISON OF D.C. AND A.C. DISTRIBUTION:  
The electric power can be distributed either by means of D.C. or A.C. Each system has its own merits and 
demerits. 
5.4.1 D.C DISTRIBUTION:  
? Advantages.  
(1). It requires only two conductors as compared to three for A.C. distribution.  
(2). There is no inductance, capacitance, phase displacement and surge problems in D.C. 
distribution.  
(3). Due to the absence of inductance, the voltage drop in a D.C. distribution line is less than the 
A.C. line for the same load and sending end voltage. For this reason, a D.C. distribution line 
has better voltage regulation.  
(4). There is no skin effect in a D.C. system. Therefore, entire cross-section of the line conductor is 
utilized.  
(5). For the same working voltage, the potential stress on the insulation is less in case of D.C. 
system than that in A.C. system. Therefore, a D.C. line requires less insulation.  
(6). A D.C. line has less corona loss and reduced interference with communication circuits.  
(7). The high voltage D.C. distribution is free from the dielectric losses, particularly in (viii) In D.C. 
distribution; there are no stability problems and synchronizing difficulties.  
 
  
 
? Disadvantages  
(1). Electric power cannot be generated at high D.C. voltage due to commutation problems.  
(2). The D.C. voltage cannot be stepped up for distribution of power at high voltages.  
(3). The D.C. switches and circuit breakers have their own limitations. 
5.4.2 A.C. DISTRIBUTION:  
? Advantages  
(1). The power can be generated at high voltages.  
(2). The maintenance of A.C. sub-stations is easy and cheaper.  
(3). The A.C. voltage can be stepped up or stepped down by transformers with ease and efficiency. 
This permits to transmit power at high voltages and distribute it at safe potentials.  
? Disadvantages  
(1). An A.C. line requires more copper than a D.C. line.  
(2). The construction of A.C. distribution line is more complicated than a D.C. distribution line.  
(3). Due to skin effect in the A.C. system, the effective resistance of the line is increased.  
(4). An A.C. line has capacitance. Therefore, there is a continuous loss of power due to charging 
current even when the line is open.  
5.5 OVERHEAD VERSUS UNDERGROUND SYSTEM: 
The distribution system can be overhead or underground.  
  Overhead lines are generally mounted on wooden, concrete or steel poles which are arranged to 
carry distribution transformers in addition to the conductors.  
  The underground system uses conduits, cables and manholes under the surface of streets and 
sidewalks.  
The choice between overhead and underground system depends upon a number of widely differing factors.  
a) Public Safety: The underground system is more safe than overhead system because all distribution 
wiring is placed underground and there are little chances of any hazard.  
b) Initial Cost: The underground system is more expensive due to the high cost of trenching, 
conduits, cables, manholes and other special equipment. The initial cost of an underground system 
may be five to ten times than that of an overhead system.  
c) Flexibility: The overhead system is much more flexible than the underground system. In the latter 
case, manholes, duct lines etc., are permanently placed once installed and the load expansion can 
only be met by laying new lines. However, on an overhead system, poles, wires, transformers etc., 
can be easily shifted to meet the changes in load conditions. 
d) Faults: The chances of faults in underground system are very rare as the cables are laid 
underground and are generally provided with better insulation.  
e) Appearance: The general appearance of an underground system is better as all the distribution 
lines are invisible. This factor is exerting considerable public pressure on electric supply companies 
to switch over to underground system.