Unit Ii
Unit Ii
Unit Ii
DISTRIBUTION FEEDERS
Design Considerations of Distribution Feeders; Radial and loop types of primary
feeders, voltage levels, feeder loading; basic design practice of the secondary distribution
system.
DC & AC Distribution Systems - Classification of Distribution Systems-
Requirements & Design features of Distribution systems-voltage drop in DC
distribution system-Radial and Ring Main Distributor. Voltage drop in AC distribution-
power factors referred to receiving end, power factors referred to respective load points.
Numerical Problems.
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A feeder includes a main or main feeder, which usually is a three-phase four-
wire circuit, and branches or laterals, which usually are single-phase or three-phase
circuits tapped off the main. Also sublaterals may be tapped off the laterals as necessary.
In general, laterals and sublaterals located in residential and rural areas are single
phase and consist of one-phase conductor and the neutral. The majority of the
distribution transformers are single phase and are connected between the phase and
the neutral through fuse cutouts.
A given feeder is sectionalized by reclosing devices at various locations in such a
manner as to remove the faulted circuit as little as possible so as to hinder service to as
few consumers as possible. This can be achieved through the coordination of the
operation of all the fuses and Reclosers.
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(II)Loop-Type Primary Feeder:
The following Figure shows a loop-type primary feeder that loops through the
feeder load area and returns back to the bus. Sometimes the loop tie disconnect switch
is replaced by a loop tie breaker due to the load conditions. In either case, the loop can
function with the tie disconnect switches or breakers normally open (NO) or normally
closed.
Usually, the size of the feeder conductor is kept the same throughout the loop. It
is selected to carry its normal load plus the load of the other half of the loop. This
arrangement provides two parallel paths from the substation to the load when the loop
is operated with NO tie breakers or disconnects switches.
A primary fault causes the feeder breaker to be open. The breaker will remain
open until the fault is isolated from both directions. The loop-type primary-feeder
arrangement is especially beneficial to provide service for loads where high service
reliability is important. In general, a separate feeder breaker on each end of the loop is
preferred, despite the cost involved. The parallel feeder paths can also be connected to
separate bus sections in the substation and supplied from separate transformers. In
addition to main feeder loops, NO lateral loops are also used, particularly in
underground systems.
The drawback of radial electrical power distribution system can be overcome by
introducing a ring main electrical power distribution system. Here one ring network of
distributors is fed by more than one feeder. In this case if one feeder is under fault or
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maintenance, the ring distributor is still energized by other feeders connected to it. In
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this way the supply to the consumers is not affected even when any feeder becomes out
of service. In addition to that the ring main system is also provided with different
section isolates at different suitable points. If any fault occurs on any section, of the ring,
this section can easily be isolated by opening the associated section isolators on both
sides of the faulty zone transformer directly.
There are additional factors affecting the decisions for primary-feeder voltage-
level selection, as shown in Figure
In general, for a given percent voltage drop, the feeder length and loading are
direct functions of the feeder voltage level. This relationship is known as the voltage-
square rule. For example, if the feeder voltage is doubled, for the same percent voltage
drop, it can supply the same power four times the distance. However the feeder with the
increased length feeds more load. Therefore, the advantage obtained by the new and
higher-voltage level through the voltage-square factor, that is,
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It has to be allocated between the growth in load and in distance. Further, the
same percent voltage drop will always result provided that the following relationship
exists:
Primary-Feeder Loading
Primary-feeder loading is defined as the loading of a feeder during peak-load
conditions as measured at the substation. Some of the factors affecting the design
loading of a feeder are
1. The density of the feeder load
2. The nature of the feeder load
3. The growth rate of the feeder load
4. The reserve-capacity requirements for emergency
5. The service-continuity requirements
6. The service-reliability requirements
7. The quality of service
8. The primary-feeder voltage level
9. The type and cost of construction
10. The location and capacity of the distribution substation
11. The voltage regulation requirements
There are additional factors affecting the decisions for feeder routing, the
number of feeders, and feeder conductor size selection, as shown in the following
Figures.
(i)Factors affecting feeder routing decisions:
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(ii) Factors affecting number of feeders:
The following Figure shows the one-line diagram of a radial secondary system. It
has a low cost and is simple to operate.
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CLASSIFICATION OF DISTRIBUTION SYSTEM
Distribution systems can be classified as follows:
i) Type of Current
Distribution system can be classified into two according to the type of current
used. These are:
(a) AC distribution system, and
(b) DC distribution system
AC distribution system: The electrical power is always generated, transmitted, and
distributed in the form of AC. The main reason for adopting the AC system over a DC
system for the generation, transmission, and distribution of electrical power is that the
alternating voltage can be conveniently changed to any desired value with the help of
transformers, since a transformer is a device used for the step-up or step down of the
voltage to the required levels. Alternating voltage can be increased to the economical
value required for transmission (high voltage) and can be reduced to a safe value for
distribution (low voltage) of electrical power. The AC distribution is classified as
primary and secondary distribution systems.
DC distribution system: Though electrical power is completely generated,
transmitted and distributed as AC. certain applications like electro-chemical works,
variable speed operation of DC motors, etc., absolutely need DC. Hence, for these
applications, AC is converted into DC at the substations and distributed either by a two-
wire or a three-wire system.
In general, AC distribution is adopted because it is simpler and cheaper than the
DC distribution system.
are used for both lighting and power loads. In case of DC supply systems, a three-wire
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Good voltage regulation is the most important factor in a distribution system for
delivering good service to the consumer. For this purpose, careful consideration is
required for the design of feeders and distributor networks.
Feeders: These are the conductors that connect substations to consumer ports and
have large current-carrying capacity. The current loading of a feeder is uniform along
the whole of its length since no tappings are taken from it. The design of a feeder is
based mainly on the current that is to he carried.
Distributors: These are the conductors, which run along a street or an area to supply
power to consumers. These can be easily recognized by the number of tappings which
are taken from them for the supply to various consumer terminals. The current loading
of a distributor is not uniform and it varies along the length while its design is largely
influenced by the voltage drop along it.
Service Main and Sub Main: The service mains are the conductors forming connecting
links between distributors and metering points of the consumer's terminal.
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RADIAL DISTRIBUTION SYSTEM
When the distributor is connected to substation on one end only with the help of
feeder, then the system is called radial distribution system. The feeders, distributors
and service mains are radiating away from the substation hence name given as radial
system. There are combinations of one distributor and one feeder, connecting that
distributor to the substation. In the Fig., distributor 1 is connected only at one end to
substation through a feeder at point A. Similarly the other feeder is feeding the
distributor 2, only at one point B.
Due to such system, if the fault occurs either on feeder or a distributor, all the
consumers connected to that distributor will get affected. There would be an
interruption of supply to all such consumers. Similarly the end of the distributor nearer
to the substation will get heavily loaded than the end which is too far away from the
substation. Similarly the consumers at the distant end of the distributor would be
subjected to the voltage variations and fluctuations, as the load on the distributor
changes. The system is advantageous only when the generation is at low voltage level
and the substation is located at the centre of the load.
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Advantages of Radial System:
The various advantages of radial system are,
1. Simplest as is fed at only one end.
2. The initial cost is low.
3. Useful when the generation is at low voltage.
4. Preferred when the station is located at the centre of the load.
The feeder in the ring fashion is divided into number of sections as AB,
BC, CD, DE and EA. The various distributors are connected at A, B, C, D
and E. Each distributor is supplied by the two feeders and hence the design is
similar to the two feeders in parallel on different paths. Hence if there is any fault on
any part of the feeder, still the consumers will keep on getting the continuous supply.
For example, if the fault occurs at point P in the section AB of the feeder, still the
consumers connected to the distributors at A and B will get supply from the sound
feeder sections AE and BC. The part AB of the feeder can be isolated and repaired. The
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feeder can be fed at one or more feeding points. Thus the disadvantages of radial system
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are eliminated in this system. The great saving in copper is another major advantage of
the ring main system.
Advantages :
Less conductor material is required as each part of the ring carries less
current than in the radial system.
Less voltage fluctuations.
It is more reliable.
Disadvantage :
It is difficult to design when compared to the designing of a radial system.
DC DISTRIBUTION
VOLTAGE DROP IN DC DISTRIBUTION SYSTEM
(I)
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(II)
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III)
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PROBLEMS
1) 2-wire d.c. distributor cable AB is 2 km long and supplies loads of 100A, 150A, 200A
and 50A situated 500 m, 1000 m, 1600 m and 2000 m from the feeding point A. Each
conductor has a resistance of 001 per 1000 m. Calculate the p.d. at each load point if a
p.d. of 300 V is maintained at point A.
SOL:
The Fig. shows the single line diagram of the distributor with its tapped currents.
2) The load distribution on a two-wire d.c. distributor is shown in Fig. The cross-
sectional area of each conductor is 027 cm2. The end A is supplied at 250 V. Resistivity
of the wire is = 178 cm. Calculate (i) the current in each section of the conductor
(ii) the two-core resistance of each section (iii) the voltage at each tapping point.
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SOL:
3) A 2-wire d.c. street mains AB, 600 m long is fed from both ends at 220 V. Loads of
20 A, 40 A, 50 A and 30 A are tapped at distances of 100m, 250m, 400m and 500 m from
the end A respectively. If the area of X-section of distributor conductor is 1cm2, find the
minimum consumer voltage. Take = 17 106 cm.
SOL:
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4) A 2-wire d.c. distributor AB is fed from both ends. At feeding point A, the voltage is
maintained as at 230 V and at B 235 V. The total length of the distributor is 200 metres
and loads are tapped off as under :
25 A at 50 metres from A ; 50 A at 75 metres from A
30 A at 100 metres from A ; 40 A at 150 metres from A
The resistance per kilometre of one conductor is 03 . Calculate :
(i) currents in various sections of the distributor
(ii) minimum voltage and the point at which it occurs
SOL:
The Fig. shows the distributor with its tapped currents. Let IA amperes be the
current supplied from the feeding point A. Then currents in the various sections of the
distributor are as shown in Fig.
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5) A 2-wire d.c. distributor 200 metres long is uniformly loaded with 2A/metre.
Resistance of single wire is 03 /km. If the distributor is fed at one end, Calculate :
(i) the voltage drop upto a distance of 150 m from the feeding point
(ii) the maximum voltage drop
SOL:
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6) A 250 m , 2-wire d.c. distributor fed from one end is loaded uniformly at the rate of
16 A/metre. The resistance of each conductor is 00002 per metre. Find the voltage
necessary at feed point to maintain 250 V (i) at the far end (ii) at the mid-point of the
distributor.
SOL:
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7) A 2-wire d.c. distributor AB 500 metres long is fed from both ends and is loaded
uniformly at the rate of 10 A/metre. At feeding point A, the voltage is maintained at
255 V and at B at 250 V. If the resistance of each conductor is 01 per kilometre,
determine :
(i) the minimum voltage and the point where it occurs
(ii) the currents supplied from feeding points A and B
SOL:
8) A 800 metres 2-wire d.c. distributor AB fed from both ends is uniformly loaded at the
rate of 125 A/metre run. Calculate the voltage at the feeding points A and B if the
minimum potential of 220 V occurs at point C at a distance of 450 metres from the
end A. Resistance of each conductor is 005 /km.
SOL:
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9) A 2-wire d.c. distributor ABCDEA in the form of a ring main is fed at point A at 220 V
and is loaded as under :
10A at B ; 20A at C ; 30A at D and 10 A at E.
The resistances of various sections (go and return) are : AB = 01 ; BC = 005 ; CD =
001 ; DE = 0025 and EA = 0075 . Determine :
(i) the point of minimum potential (ii) current in each section of distributor
SOL:
The Fig.(i) shows the ring main distributor. Let us suppose that current I flow in
section AB of the distributor. Then currents in the various sections of the distributor are
as shown in Fig (i).
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The actual distribution of currents is as shown in Fig. 13.37 (ii) from where it is
clear that C is the point of minimum potential.
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10) A single phase a.c. distributor AB 300 metres long is fed from end A and is loaded as
under :
(i) 100 A at 0707 p.f. lagging 200 m from point A
(ii) 200 A at 08 p.f. lagging 300 m from point A
The load resistance and reactance of the distributor is 02 and 01 per kilometre.
Calculate the total voltage drop in the distributor. The load power factors refer to the
voltage at the far end.
SOL:
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11) A single phase distributor one km long has resistance and reactance per conductor
of 01 and 015 respectively. At the far end, the voltage VB = 200 V and the current is
100A at a p.f. of 08 lagging. At the mid-point M of the distributor, a current of 100 A is
tapped at a p.f. of 06 lagging with reference to the voltage VM at the mid-point.
Calculate :
(i) voltage at mid-point
(ii) sending end voltage VA
(iii) phase angle between VA and VB
SOL:
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