1. The document provides an overview of a training module on overhead line work. It covers power system structure, design principles of distribution lines, and installing/maintaining electrical equipment. 2. The objectives are to address distribution line problems, energize 33kV lines, develop awareness of installing/maintaining 33kV lines, and discuss insulation and equipment selection. 3. The target group are trainees in categories S1-S4 and W3-W6.

1. Addis Ababa Science and Technology University
Electrical and Computer Engineering Department
OVERHEAD LINE WORK TRAINING
Prepared by: DemsewM.
May 2017

2. Module Contents
o Introduction to power system structure.
o Design principles of primary distribution voltage line.
o Inspect overhead structures,
o Installing and maintaining line and electrical
equipment,
o Maintain and energized LV conductors,

3. Module Objective
o To address main problem exist in overhead distribution line
o To energize 33kV overhead distribution line
o To develop general awareness about how to install and
maintain 33kV overhead distribution line.
o To develop design principle idea of 33kV distribution line
o Distribution voltage level selection criteria
o Selection of distribution system equipment’s
o To discuss insulation in 33kV distribution system.

4. Module Category/Target Group
Prepared for EEP trainee category of:
oS1-S4, and
oW3-W6

5. CHAPTER ONE
Power System Structure
Chapter Contents:
–Power Generation System
–Power Transmission System
–Power Distribution System

6. Structure of Power System

7. 1. Electric Power Generation
• Electric generators are devices that convert energy
from a mechanical form into an electrical form.
• This process, known as electromechanical energy
conversion, involves magnetic fields that act as an
intermediate medium.
• The input to the generating machine can be derived
from a number of energy sources.

8. Hydro Electric Power Generation

9. Group Discussion
• How electric generation is possible from run-
off river and ocean wave?

10. Solar Power Generation
Photovoltaic energy -
solar energy converted
directly to electrical
current

11. Group Discussion
• How solar thermal electric generation is
possible?

12. Bio-Electricity

13. • This energy
source involves
the use of high-
pressure, high-
temperature
steam fields that
exist below the
earth’s surface.
Geothermal Power Generation

14. Wind Power Generation
• Wind power - advantages and disadvantages
• Wind farms - potential exists in Great Plains, along seacoasts and Eastern
Washington
http://www.awea.org/projects/washington.html

15. Group Discussion
• Discuss the situation of our current power
generation from wind?
• Discuss also other source of electricity
generation?

16. 2. Electric Power Transmission
• Electric power transmission is the bulk
transfer of electrical energy, a process in the
delivery of electricity to consumers.
• A power transmission network typically
connects power plants at remote location to
multiple substations near a populated area.

17. Cont’d…
• Overhead electric power transmission allows
distant energy sources (such as hydroelectric
power plants) to be connected to consumers in
population centers, and
• May allow exploitation of low-grade fuel
resources such as coal that would otherwise be
too costly to transport to generating facilities.

18. Cont’d…
• Today, transmission-level voltages are usually considered to be
132 kV and above (1600kV, 1100kV, 800kV, 500kV, 400kV and
230kV).
• Voltages above 230 kV are extra high voltage and require
different designs compared to equipment used at lower voltages.
• Lower voltages such as 66 kV and 33 kV are usually considered
sub-transmission voltages but are occasionally used on long
lines with light loads.
• Voltages less than 33 kV are usually used for distribution.

19. Cont’d…

20. Overhead Transmission Structures
The basic overhead transmission line structure:
o Bare conductor
oWood, Concrete, Lattice or tabular steel pole
o Insulator (Suspension , Post or Pin type)
o Ground Wire

21. Bare Conductor
• The most widely used conductor material for
power transmission and distribution are:
o Aluminum and,
o Copper
• Due to their:
o Electrical conductivity
o Weight, Strength and Durability
o Cost and
o Installation flexibility

22. Cont’d…
• Aluminum conductors reinforced with steel (known as
ACSR) are primarily used for medium and high voltage
lines and may also for overhead services to individual
customers.
• Aluminum conductors has the advantage of better weight
than copper, as well as being cheaper.
• Some copper cable is still used, especially at lower
voltages and for grounding.

23. Cont’d…
• While larger conductors may lose less energy due
to lower electrical resistance, they are more costly
than smaller conductors.
• An optimization rule called Kelvin's Law states
that the optimum size of conductor for a line is
found when the cost of the energy wasted in the
conductor is equal to the annual interest paid on
that portion of the line construction cost due to the
size of the conductors.

24. Cont’d…
• F(conductor size)= Cost of conductor as a
function of its size
• P𝒍𝒐𝒔𝒔=The power loss on the conductor
• Cost of P𝒍𝒐𝒔𝒔= the cost of the lost energy on the
conductor in $/KWH
• Y(conductor size)=the annual interest paid on that
portion of the line construction cost due to the size
of the conductors.

25. Cont’d…
• Therefore, the optimization problem providing
the optimum size of the conductor is given by;
Optimization [F(conductor size)]
When
Y(size of conductor)=Cost of P𝒍𝒐𝒔𝒔
So,
Conductor Size=optimum size of conductor

26. Cont’d…
• Bundled conductors are used for voltages over 200 kV to avoid
corona losses and audible noise.
• Bundle conductors consist of several conductor cables connected
by non-conducting spacers. For 220 kV lines, two-conductor
bundles are usually used, for 380 kV lines usually three or even
four.
• Spacers must resist the forces due to wind, and magnetic forces
during a short-circuit.

27. Cont’d…
• Overhead power lines are often equipped with a ground
conductor (shield wire or overhead earth wire).
• A ground conductor is a conductor that is usually grounded
(earthed) at the top of the supporting structure to minimize the
likelihood of direct lightning strikes to the phase conductors.
• Very high-voltage transmission lines may have two ground
conductors.
• Shield wires on transmission lines may include optical fibers
(OPGW), used for communication and control of the power
system.

28. Cont’d…
• These are either at the outermost ends of the highest cross beam, at
two V-shaped mast points, or at a separate cross arm. Older lines may
use surge arrestors every few spans in place of a shield wire.

29. Conductors
1. Suspended wires for electric power transmission are
bare, except when connecting to houses, and are
insulated by the surrounding air. Cooper has been
replaced by Aluminum with steel wire core to provide
the material strength.
1. Underground cable – insulated
1. Sea cable – insulated

30. Electric Pylon
• Structures for overhead lines take a variety of shapes depending
on the type of line.
• Structures may be as simple as wood poles directly set in the
earth, carrying one or more cross-arm to support conductors, or
"armless" construction with conductors supported on insulators
attached to the side of the pole.
• Tubular steel poles are typically used in urban areas.
• High-voltage lines are often carried on lattice-type steel towers.

31. Cont’d…
• For remote areas, wood pole or Concrete poles have been used.
• Each structure must be designed for the loads imposed on it by
the conductors (High voltage or Low voltage structure).
• Foundations for tower structures may be large and costly,
particularly if the ground conditions are poor, such as in
wetlands.
• Each structure may be considerably strengthened by the use of
guy wires to resist some of the forces due to the conductors.

32. Anchor pylons or strainer pylons
are employed at branch points as
branch pylons and must occur at a
maximum interval of 5 km, due to
technical limitations on conductor
length
Branch pylon
is a pylon that
is used to start
a line branch.
Tension tower for
phase transposition
Type of pylon by function

33. Types of Pylon by Conductor
Arrangements
Single-level pylon Two-level pylon Three-level pylon

34. Insulators
• Insulators must support the conductors and withstand both the
normal operating voltage and surges due to switching and lightning.
• Insulators are broadly classified as either pin-type, which support
the conductor above the structure, or suspension type, where the
conductor hangs below the structure.
• Up to about 132 kV both types are commonly used. At higher
voltages only suspension-type insulators are common for overhead
conductors.
• Insulators are usually made of ceramic or reinforced glass.

35. Cont’d…
• Suspension insulators are made
of multiple units, with the
number of unit insulator disks
increasing at higher voltages.
• The number of disks is chosen
based on line voltage, lightning
withstand requirement, altitude,
and environmental factors such
as fog, pollution, or salt spray.

36. Bushings
• Bushings are specific
insulators that required
where the wire enters
buildings or electrical
devices, such as
transformers or circuit
breakers, to insulate the
wire from the case.
• They are hollow
insulators with a
conductor inside them.

37. Surge Arrester
• Surge arresters are
used to avoid damage
to the electrical
equipment through
controlled conduction
of the excess voltage
(ex. Lightning or
switching surges)
through the arrester
itself by grounding.

38. Design Principle of OH Transmission Line
• Design of these lines requires:
o Minimum clearances
o High insulation level
o Economical transmission voltage level
o Optimum conductor size
o Enough mechanical strength of supporting tower.
o Optimum span length

39. Group Discussion
1. How you evaluate the transmission system in
Ethiopia?
a. The power quality issue
b. The power loss problem
c. The voltage drop problem

40. 3. Electric Power Distribution
• In distribution systems the supply authority
collects the bulk energy at 66 kV or less from the
transmission substation.
• There are specific voltage values used in the
distribution of electrical power. These voltage
values, which are all ‘line to line’ values are
66kV, 33kV, 15kV, 11kV, 6.6kV, 3.3kV and
400/230V.

41. Cont’d…
• Electric power distribution consists of two main
elements:
o the retail function, or buying and selling of electrical energy and,
o the distribution function, which is the transport and
dissemination of reliable power to the customer.
• As electricity is not ‘stored’, many aspects of
distribution are influenced by minimizing the costs
associated with the (instantaneous) ‘buy and sell’
operation.

42. Distribution Voltage Selection
• The choice of voltage to be used on any particular section
in the distribution system will be influenced by:
– Decisions associated with voltage drops resulting from
large current loads
– Capital cost of transformers used to change voltage
levels
– Capital costs of construction of distribution lines and
associated switchgear to operate at the chosen voltage
– Environmental aspects of the system installation.

43. Cont’d…
• For any given electrical load (in kVA), the higher the load
voltage the lower will be the resultant current required.
• As it is the current flowing in the supply cabling which
creates the voltage drop and the heating (I2R) loss, to
minimize losses we try to keep the values of line current
as low as possible.
• In this way the necessary distribution line voltage level
can be determined, along with the resultant cost of
constructing the line.

44. Types of Distribution Feeder
a. Radial Feeder
• Many distribution systems operate using a ‘radial
feeder’ system.

45. Cont’d…
• Radial feeders are the simplest and least expensive,
both to construct and for their protection system.
• This advantage however is offset by the difficulty
of maintaining supply in the event of a fault
occurring in the feeder.
• A fault would result in the loss of supply to a
number of customers until the fault is located and
cleared.

46. Cont’d…
b) Parallel Feeders
• A greater level of reliability at a higher cost is achieved with a
parallel feeder.

47. Cont’d…
c) Ring Main Feeder
• A similar level of system reliability to that of the parallel
arrangement can be achieved by using ‘ring main’ feeders.

48. Cont’d…
• This is for a huge growing load supplied by a parallel feeder
where the cabling has been installed along different routes
and most common in urban and industrial environments.
• If a fault occurs on a feeder cable at any point around the ring, the
faulty section may be isolated by the operation of the protecting
circuit breakers, at the same time maintaining supply to all
substations on the ring.
• The system have complex protection and control system and also
costy to construct as compared with radial network.

49. Cont’d…
d) Meshed Systems
• In transmission and sub-
transmission systems,
usually parallel, ring or
interconnected (‘mesh’)
systems are used. This
ensures that alternative
supply can be made to
customers in the event of
failure of a transmission
line or element.
Mesh=parallel+ring

50. Group Discussion
1. Discuss about the power distribution situation in
Ethiopia?
a. Types of distribution feeder?
b. How you rate the distribution reliability issue in
your area?
c. Frequently facing fault type?
d. Mechanism of location of fault and its
clearance?
e. Major cause of distribution power interruption?

51. CHAPTER TWO
Design of Overhead Distribution System
The designing process contain:
o Deciding distribution voltage level.
o Choosing the economic size of conductor.
o Proper selection of insulator
o Deciding the reasonable span length
o Selection of pole size.

52. Pre-Line Design Considerations
• Basic requirements to be considered when designing OH
distribution lines:
(a) Potential number of Customers and total load(Demand and Peak
Load);
(b) Estimation of potential load growth;
(c) Selection of Voltage for line operation;
(d) Size and location of transformers
(e) Selection of Route
(f) Length of line
(g) Life Cycle costs

53. Design Principle
• The main technical aspects in the design of
overhead lines are ensuring that:
o The mechanical load forces do not exceed the strength
of structures or other components,
o There are adequate clearances – between the
conductors and ground or from other objects in the
vicinity of the line.

54. Loading on OH Structures
• The loads on a structure consist of three mutually
perpendicular systems of load acting normal to
the direction of line, and parallel to the direction
of the line. These loads can be described as:
oVertical load
oTransverse load
oLongitudinal load

55. Distribution Voltage Level Selection

56. Optimum Voltage level

57. Cont’d…
• Generally the demand and the line length or distance
between customer and nearby substation will be
given; the most economic voltage can be determined
by the following equation.
Where, L = Customer distance in mile.

58. Economic Size of Conductor
• Let us consider Kelvin’s Law in the following
formula to determine the optimum size of
conductor.
Where,
C = most economical density of current (Ampere/mm2)
a = percent annual expense to the construction cost of conductor
p = price of conductor ($/kg)
q = cost of electricity ($/kWh)

59. Cont’d…
• The current I is calculated as follows:
Where,
µ = utility factor being (0.6 )
pf = power factor being 0.85
V = line voltage (kV)
P = Maximum Power (kW)
The most economic size of the conductor is, A=I/C (mm2)

60. Example 2.1
• Consider a rural community to be electrified have a
total load of 50MVA. The community load increase
10% annually and the community is 35 mile from the
near by substation.
a) Determine the economic distribution voltage level
for the community accounting a 5 year future load.
b) Determine the most economical conductor size to
transmit the power in part-a?

61. OHL Insulation Consideration
• The target withstand voltage for an insulator design can
be calculated with the following Equation,
• The design withstand voltage of each insulator discs is proportional
to the number of insulator strings.
𝐍 =
𝐓𝐚𝐫𝐠𝐞𝐭 𝐰𝐢𝐭𝐡𝐬𝐭𝐚𝐧𝐝 𝐕𝐨𝐥𝐭𝐚𝐠𝐞 (𝐊𝐕)
𝐃𝐢𝐞𝐥𝐞𝐜𝐭𝐫𝐢𝐜 𝐬𝐭𝐫𝐞𝐧𝐠𝐭𝐡 𝐨𝐟 𝐭𝐡𝐞 𝐢𝐧𝐬𝐮𝐥𝐚𝐭𝐨𝐫 (𝐊𝐕/𝐝𝐢𝐬𝐜)
Where N is the number of disc insulators connected to withstand the target voltage.

62. Example 2.2
• Set an appropriate insulation for Example 2.1
using the following insulator:
a) Ceramic insulator of dielectric strength 12kV/disk
b) Glass insulator whose dielectric strength is 8kV/disk

63. Span Length
• Consider the line which is routed between pole-A and
pole-B and subjected to a stringing tension force, T and a
vertical force due to the weight of the conductor, w. The
sag of the conductor is given by the following Equation:
• D= conductor sag
• L= span length
• T= Stringing Tension force
• W=weight of conductor per unit length

64. Cont’d…
• The length of the
conductor which has a
maximum sagging
distance of d can be
expressed as using the
following equation:

65. Example 2.3
• Given that:
oW=25N/m
oT=1200N
oPole height=13m
oMinimum clearance required=8.5m
• Determine the optimum span length and the
conductor length required to reach the customer
terminal for the sytem given in Example 2.1?

67. CHAPTER FOUR
LINE EQUIPMENT OF 33KV
Besides poles, conductors and insulators, many other pieces of
equipment are necessary to get electric power from the substation to a
consumer.
– Distribution Transformer
– Fuses
– Lightning or Surge Arresters
– Capacitors
– Switches

68. Distribution Transformer
• The distribution
transformer is the most
important of these pieces
of equipment.
• It would be impossible
to distribute power over
such long distances.
• The purpose of a
distribution transformer
is to step down voltage.

69. Cont’d…
• As per Faraday's law
of electromagnetic
induction,

70. Cont’d…
• Most distribution transformers consist of:
oA closed-loop magnetic core on which are
wound two or more separate copper coils,
oA tank in which the core coil assembly is
immersed in cooling and insulating oil,
oBushings for bringing the incoming and
outgoing leads through the tank or cover.

71. Bushings
• A bushing is an insulating lining for the
hole in the transformer tank through which
the conductor must pass.
• On every distribution transformer,
attachments are to be found which are
normally referred to as primary bushings
and secondary bushings.
• There are three types of bushings; the solid
porcelain bushing, the oil-filled bushing,
and the capacitor type bushing.

72. Tap Changer
• It is often necessary to vary the
voltage in a transformer winding
(primary) to allow for a varying
voltage drop in the feeder
(transmission) lines.
% 𝐓𝐚𝐩 𝐂𝐡𝐚𝐧𝐠𝐞 =
𝑽 𝒏𝒐𝒎𝒊𝒏𝒂𝒍− 𝑽 𝒐𝒑𝒆𝒓𝒂𝒕𝒆𝒆𝒅
𝑽 𝒏𝒐𝒎𝒊𝒏𝒂𝒍
∗ 𝟏𝟎𝟎%

73. Fuse
• A fuse consists of a
short piece of metal
having low melting
characteristics which
will melt at a rated
temperature.
• The fuse melt when a
current above its rated
limit flow through it
and interrupts the
circuit.

74. Surge Arresters
• A lightning arrester is a
device that protects
transformers and other
electrical apparatus
from voltage surges.
• These surges can occur
either because of
lightning or improper
switching in the circuit.

75. Cont’d…
• The lightning arrester provides a path over which the surge
can pass to ground as before it has a chance to attack and
seriously damage the transformer or other equipment.
• There is usually an air gap in series with a
resistive element, and whatever the resistive (or
valve) element is made of, it must act as a
conductor for high-energy surges and also as an
insulator toward the line energy.

76. Cont’d…

77. Capacitors
• The voltage on a circuit fall below a specified
level for some reason, a device called a capacitor
can momentarily maintain the voltage at line
value.
• It is the job of capacitors to keep the power factor
as close to 1 as possible.
• Keeping the power factor close to 1 is a
considerable economic advantage to the utility
company and to the consumer.

78. Cont’d…
• The capacitor usually consists of
two conductors separated by an
insulating substance.
• It can be made of aluminum foil
separated by oil-impregnated
paper, or synthetic insulating
materials.
• Capacitance depends on the area
of the conductors, on the
distance between the conductors
and on the type of insulating
material used.

79. Cont’d…

80. Switches
• Switches are used to interrupt the continuity of a circuit.
• They fall into two broad classifications: air switches and oil
vacuum or gas (SF6) switches.
• As their names imply, air switches are those whose contacts
are opened in air, while the other type switches are those
whose contacts are opened in oil, vacuum, or gas.
• Oil switches are usually necessary only in very high-voltage,
high-current circuits.