Power Dist. System

1. Power Distribution and
Utilization

2. Books
 Power System Analysis by John J Grainger & William D.
Stevenson, McGraw Hill, 1994.
 Electrical Power Distribution Engineering by Tarun
Gonen, CRC Press, 2014.
 Principles of power systems by V.K mehta and R. Mehta,
3rd edt. 2005
2

3. Grading Policy
 Mid Term:30
 Final Term: 40
 *Sessional Marks: 30
 Total: 100
*Sessional marks are to be given on the basis of quizzes &
assignments/class performance.
3

5. 1-When electricity leaves a power plant
2- its voltage is increased at a “step-up” substation
3- Next, the energy travels along a transmission line to the area where the power is needed
4- Once there, the voltage is decreased or “stepped-down,” at another substation
5- and a distribution power line carries the
6- electricity until it reaches a home or business areas.
How Electricity is delivered to
us?

6. Distribution System
Power Distribution
System
is that part of an electric
power system which is
dedicated to delivering
electric energy to end
users. It is part of the
system between
transmission and the
consumer service point.

7. Power Distribution
Power distribution
System is the penultimate
stage in the delivery
(before retail) of
electricity to end users. It
is generally considered to
include medium-voltage
(less than 50 kV) power
lines, electrical
substations and pole-
mounted transformers,
low-voltage (less than
1000 V) distribution wiring
and sometimes electricity
meters.

9. Power Distribution System Development
In the early days of electricity distribution, direct current DC
generators were connected to loads at the same voltage. The
generation, transmission and loads had to be of the same voltage
because there was no way of changing DC voltage levels, other than
inefficient motor-generator sets. Low DC voltages were used (on the
order of 100 volts) since that was a practical voltage for incandescent
lamps, which were then the primary electrical load. The low voltage
also required less insulation to be safely distributed within buildings.
The losses in a cable are proportional to the square of the current, the
length of the cable, and the resistivity of the material, and are
inversely proportional to cross-sectional area. Early transmission
networks were already using copper, which is one of the best
economically feasible conductors for this application. To reduce the
current and copper required for a given quantity of power transmitted
would require a higher transmission voltage, but no convenient
efficient method existed to change the voltage level of DC power
circuits. To keep losses to an economically practical level the Edison DC
system needed thick cables and local generators. Early DC generating
plants needed to be within about 1.5 miles of the farthest customer to
avoid the need for excessively large and expensive conductors.

10. Power Distribution System
Development
The adoption of alternating current (AC) for electricity
generation dramatically changed the situation. Power
transformers, installed at power stations, could be used
to raise the voltage from the generators and
transformers at local substations reduced it to supply
loads. Increasing the voltage reduced the current in the
transmission and distribution lines and hence the size
of conductors required and distribution losses incurred.
This made it more economical to distribute power over
long distances. Generators (such as hydroelectric sites)
could be located far from the loads.

11. Systems of Distribution Lines
According
to
Voltage
According
to
Current
According
to
Service
According
to
Construction
According
to
No. of Wires
According
to
Connection
Scheme
Secondary
Distribution
Primary
Distribution
AC
Distribution
DC
Distribution
Industrial
Domestic Underground
Overhead
3 Wire
2 Wire 4 Wire
Ring
Mains
Radial
Inter-
Connected

12. Overhead and Underground
(According to Construction)
The distribution system can be either overhead or
underground. It is usually overhead, though for higher
load densities in cities or metropolitan areas, it is
underground. The choice between overhead and
underground depends upon a number of number of
widely differing factors, such as the;
 importance of service continuity,
 improvement in appearance of the area,
 comparative annual maintenance cost,
 capital cost
 and useful life of service

14. Overhead System
In this system, long RCC or steel towers/poles are
erected on the surface of earth, and, with the help of
cross arms and insulators, naked wires are run overhead
at a height, not less than 10m from ground. overhead
lines are exposed to trees, animals and birds , to wind
and lightning, and to traffic and kites, therefore this
system has less reliability.

15. Mechanical Components of O.H Lines:
 Line Supports
 Line Insulators
 Line Conductors
 Miscellaneous components like cross-
arms, earth wire, ground wire, bird
guard, stay wire, lightning arrestor, anti-
climbing devices, phase-plates and danger plates etc.

16. Line supports
Line supports, as the name suggest, support the O.H line, cross-arms,
insulators, conductors, earth wire, etc. They should therefore be
mechanically strong, cheap and light in weight for easy transport and long
life. Various types of
(a) Wooden Poles
(b) RCC/PCC Poles
(c) Steel Poles

17. Overhead System
Wooden Poles:
Wooden poles are made of impregnated sal/chir/teak,deodar/kail wood.
The poles are generally 10-11m in length with sufficient base. They may
be of simple design (Fig.1) or having shape ‘A’ (Fig.2) or having shape ‘H’
(Fig.3). The ‘H’ type is stronger.

18. Fig.1 Fig.2 Fig.3

19. Wooden Poles
The wooden poles may be used upto 11KV. Their use is limited to rural
areas. These poles do not need earthing but their life is short. The
maximum span, for which they are used, is 150 meters. Usually aluminium
cap is provided on the top of these poles to protect the end grains.

20. RCC Poles
The reinforced cement concrete poles are very
much in use in rural as well as urban areas.
(See Fig.4).They are, generally, of square cross
section which goes on reducing towards the
top. They are also 10-11 maters long and are
used up to 33KV with span of 250m.They do not
need earthing. They are more expensive than
wood poles but cheaper than steel poles. They
are preferred where decorative consideration is
predominant Their life is long but have
shattering tendency when hit by a vehicle.

21. Fig.4

22. Steel Poles
Steel poles are stronger than RCC poles and, therefore, have a long life,
up to 50 years or more, if they are regularly painted and protected against
atmospheric reaction.
They may be
 Rail Poles (Rail Track shape)
 Tabular Poles (Round)
 Rolled steel joint

23. Steel Poles
They are 10-13 meter length and are used upto 33KV
distribution purposes. Their cost is high. They need
earthing and precaution against electric shocks.
Generally they are not buried in in the ground but are
set in concrete foundations.

24. Line Insulators
Line insulators are installed on line supports. These help in avoiding direct
contact of the conductors with poles. Thus, any leakage through the
support to the earth is stopped, and the risk of electric shock from poles is
also minimized.
On the basis of material used, they are of two types.
(1) Porcelain
(2) Glass

26. Comparison of Porcelain and Glass
Insulators
Porcelain Glass
1. It is mechanically stronger It is not strong; it is fragile
2. Its surface may be glazed to
prevent accumulation of water, dust
and dirt, and, thus , surface leakage
can be minimized.
The surface cannot be glazed; it may,
therefore, cause surface leakage.
3. Internal cracks or faults cannot be
easily detected as it is not transparent.
It is transparent; even visual
inspection can detect internal
cracks/faults
4. Its dielectric strength is about
70KV/cm
Its dielectric strength is about
150KV/cm
5. It can be used at any high voltage. Its use is limited upto 33KV.

27. On the basis of construction , main types are as follows
(1) Pin insulators
(2) Shackle Insulators
(3) Disc or suspension type Insulators
(4) Guystrain or Egg type or Stay Insulators
(5) Strain or tension type Insulators

28. Pin type insulators
These insulators are mounted on the pole with the help of pin, hence, the
name. they have a groove at their top, and the conductor is tied in the
groove by a binding wire going through its neck.
These are manufactured for voltages up to 33KV and are cheaper than
other types.

30. Shackle Insulators
Shackle insulators are used on low tension (L.T) lines up to 1000V. They
are fixed with the pole though nut and bolt, with the help of a clamp D-
strap. They can be used in horizontal as well as vertical positions. The
conductor is fixed on its neck and tied with a binding wire.

32. Disc or Suspension type Insulator
They are used as insulators on high voltage lines for suspension and dead
ending. They can be assembled with each other through ball and socket
joints to form string assemblies to suit a particular voltage level. Another
advantage of string is that, in case of fault in any disc, they whole string
need not to be replaced, only faulty piece is replaced.

34. Guystrain or Egg type or Stay
Insulators
They are of egg shape and are used on stay wires. Therefore, they are
also, sometime, called stay Insulators. Their purpose is to protect human
beings and animals against shock through stay wires. They are used
wherever it is not possible to earth the stay wires or guard wires.

37. Strain or Tension Type
Insulators
Strain or tension type insulators are designed for
handling mechanical stresses at angle positions where
there is a change in the direction of the line or at
termination of the line.

38. Line Conductors
The conductors used distribution of power should have
the following properties:
1. They should have low resistivity (or high
conductivity) in order to carry more current.
2. They should have high mechanical (or tensile)
strength to withstand mechanical loads like ice,
stones, wind etc., otherwise they may sag.
3. They should be corrosion free.
4. They should be light in weight.
5. They should be cheap.
6. They should cause minimum voltage drop (IR) and
power loss (I2R) .
7. They should be flexible. This is the reason that
stranded conductors are preferred.

39. Types of Conductors
In view of mentioned requirements, the following types of line conductors
are used.
(1) Copper
(2) Aluminium
(3) A.C.S.R
(4) Galvanized Steel

40. Copper Conductors
Copper conductors give a relatively high tensile strength. It has a good
electrical conductivity, long life, and high scrape value. Copper
conductors is most suitable for distribution work where spans are short.
Their only negative point is that it is costly.

41. Aluminium Conductors
Aluminium is the next best conducting material after
copper. Its conductivity is less (2/3rd of copper) but it is
lighter in weight(1/3rd of copper). It is more effected
by sag due to weak mechanical strength. With
aluminium conductors, the length of span is also
reduced. However, it is very much in use in place of
copper due to low costs. Several types of aluminium
conductors are as follows.
(a) AAC (All-Aluminium Conductors)
(b) ACSR (Aluminium conductors Steel Reinforced)
(c) AAAC (All-Aluminium Alloy Conductors)
(d) ACAR (Aluminium Conductors, Alloy Reinforced)

42. AAAC
AAC

43. ACSR
Due to less availability of aluminium and also for
reducing cost, ACSR(Aluminium Coductor with Steel
reinforced) has become very popular now.It has core of
galvanized(to prevent corrosion) steel surrounded by
aluminium strands.

45. Galvanized Steel
Due to high tensile strength, galavanized steel can be
used at extremely large spans.Galvanization protects
steel against corrosion.They are more in use in rural
areas where cost is the main consideration.They are
also used as earth wire on poles, as stay wires and have
other similar applications

46. Miscellaneous components
Cross arms:
These are the wooden arms (sisso or sal) of two poles which
supports insulators.
Earth wire:
This wire runs at the top of the pole. It must be properly
earthed. It protects against lightning.
Guard Wire:
These wires are provided above or below power lines
crossing the telecommunication lines.
Bird Guard:
It is an ebonite stick, with round top and fixed near the
insulator insulators to prevent flashover due to birds sitting on the
conductors. This protection is provided only with pin insulators.
Phase Plates:
One each phase, R, Y,B marked plates are put so that
different plates may be identified.
Danger Plates:
A danger plate, marked with the line voltage, is fastened on
poles as a caution to the public

47. Underground System
In this system, properly insulated cables re run
in pipes, underground. It needs lot of earth
work as well as heavy insulation on the wires.
However cost of poles, cross arms and
insulators are eliminated. underground system
is more reliable than O.H system. Unlike
overhead distribution, underground lines
reduce hazards to the general public, improve
the aesthetics of our landscape and aren’t
exposed to natural weather forces that can
decrease efficiency and the longevity of your
electric system. Underground system is
expensive than O.H system.

48. Comparison b/w O.H and U.G systems
O.H System U.G System
1.The O.H system is laid on the
ground, it disturb public life.
It is laid underground, it does not
disturb public life.
2.It gives a shabby look and
damages the beauty of
surrounding.
It maintains the beauty of
surrounding.
3. Repair and extension is easy Repair and extension is difficult
4.It has high cost of maintenance Maintenance cost is low.
5.It disturb the communication as
well as transport system.
6.It does not disturb the
communication and public
transport system.

49. Comparison b/w O.H and U.G systems
O.H System U.G System
6.It can work up to 400KV. It is not used above 66KV due to
insulation problems,
7.It needs protection against
lightning surges.
It does not need any protection
against lightning surges.
8.Fault occurs frequently. Very little chance of fault exists.
9.Initial cost is high. Its initial cost is higher.
10.Interruption of supply is
common.
It has very little chance of
interruption of supply.
11.It is flexible It is less flexible.

50. Thank you

51. Transmission Sub-Station

52. Steel Transmission Tower

53. Distribution system lines
and associated equipment

54. Service line to residence

55. Secondary to underground
service via a riser