Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3 (4): 627-630
© Scholarlink Research Institute Journals, 2012 (ISSN: 2141-7016)
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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(4) 627-630 (ISSN: 2141-7016)
Effect of Sag on Transmission Line
Oluwajobi F. I., Ale O. S. and Ariyanninuola A.
Department of Electrical and Electronics Engineering Technology
Rufus Giwa Polytechnic Owo, Ondo State, Nigeria.
Corresponding Author: Oluwajobi F. I.
___________________________________________________________________________
Abstract
This paper looks into the factors affecting sag in conductors while erecting an overhead line, it is very important
that conductors are under safe tension. If the conductors are too much stretched between supports, the stress in
the conductor may reach unsafe value and in certain cases the conductors may break due to excessive tension.
The conductor sag should be kept to a minimum in order to reduce the conductor material required and to avoid
extra pole height for sufficient clearance above ground level. It is also desirable that tension in the conductor
should be low to avoid the mechanical failure of conductor and to permit the use of less strong supports. The
effects of sag on electrical construction were examined. Also, a look into the solution to effect of sag were
examined. The meaning sag was not left out. This write up suggested ways to reduce the effect of sag on
overhead line conductors.
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Keywords: conductor, electrical, line, over head, tension, transmission, sag.
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INTRODUCTION
Supporting Tower: These are towers carrying
Transmission lines do not connect between their
electricity from generating stations to businesses,
supporting towers in a straight line. The shape
homes, and schools. They are normally constructed
formed by a line strung between two supports is
of steel or concrete because of the heavy cables they
called a catenary. [Elizabeth Eng – 2004] If there is
bear, whereas the power lines in cities are usually
too much tension, the sag will be too little and the
supported on wooden poles or placed underground.
line can snap. However, if there is too much sag, it
will increase the amount of conductor used,
increasing the cost more than is necessary. The more
space there is between transmission towers, the more
the transmission line will sag. Sag is provided in
transmission lines in order to lessen the tension of the
transmission lines. Sag literally means to be bending
in shape [Seppa T O, 1992]
DEFINITION OF TERMS
Electric-Power Transmission: This is the bulk
transfer of electrical energy, from generating power
plants to electrical substations located near demand
centers. Power transmission lines are used to
transport electrical energy over long distances.
Transmission Line: "Transmission lines" refers to
electrical transmission lines or cables such as
telephone lines and power lines.
Tension: in physics, tension is (magnitude of) the
pulling force exerted by a string, cable, or
similar solid object on another object.
Sag: is defined as the vertical distance between the
point where the line is joined to the tower and the
lowest point on the line. [www.eng.uwi.tt]
Transmission Tower: is a tall structure, usually a
steel lattice tower, used to support an overhead power
line. They are used in high-voltage AC and DC
systems, and come in a wide variety of shapes and
sizes.
Fig. 1. Diagram showing the definition of sag
Conductor: These are materials that allow free flow
of current and heat. In other words it allow the
passage of heat and electricity
Insulator: These are materials that do not allow the
free flow of heat and electricity on them. They are
used in transmission to insulate the conductor from
ground and tend to be made of either glass or
porcelain and in some cases, ceramic.
Span: Span is the horizontal distance between two
towers
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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(4) 627-630 (ISSN: 2141-7016)
WHAT IS SAG?
Sag literally means to be bending in shape. In
overhead transmission lines, the difference in level
between points of supports (towers or utility poles)
and the lowest point on the conductor is called sag.
are under safe tension. If the conductors are too much
stressed between the supports (towers, utility poles),
then the stress on the conductors may reach to an
unsafe level and the conductor may break due to
excessive pressure (i.e. tension). In order to permit
safe tension in the conductors, the conductors (i.e. the
transmission lines) are not fully stretched but are
allowed to have a dip or sag.
It Increases the Cost in Transmission Line when
Too Much: The more space there is between the
transmission towers, the more the transmission line
will sag. If there is too much sag in a transmission
line, it will increase the amount of conductor used,
increasing the cost more than is necessary.
Fig. 2: Sag representation
It Causes Power Failure: When a transmission line
sag excessively, it is liable of causing power failure.
An overheating electrical transmission line sagging
into a tree sparked the greatest power failure in the
Western United States in 1996. A similar incident is
suspected to have caused the recent East Coast
blackout.
ARCHITECTURE
There are two subdivisions of a transmission line,
with which each end of these subdivisions are
terminated by a tension tower. Between these tension
towers 5 to 10 supporting towers are used to carry the
phase conductors. At the supporting tower insulators
are positioned vertically and are aliened with the
conductor angle of the tension tower. With respect to
load conditions, the tension tower carries both
vertical and horizontal loads while the supporting
tower carries only the vertical loads. This
construction has the advantage that damages that can
be impacted by storm is limited to one section.
FACTOR AFFECTTING SAGGING IN A
TRANSMISSION LINE
The factors that affect sagging of transmission
lines are:
Temperature: As the temperature increases, the
outstretched conductor length will increase by an
amount equal to:
ΔL =αTS…………………………..(i)
α = the coefficient of thermal expansion
T = the temperature increase in oC
S = the span length in meters
Why is Sag provided in the Transmission Line?
The sag is as a result of the tensioning of the line and
must not be too low otherwise the safety clearances
may not be met. Also, the sag had to be such that it
caters for ice loading in the winter of temperate
climates. If the sag is large, and the line becomes
heavily loaded, then the sag will further increase and
breach the safety clearances. Similarly, if the sag is
low, then when the line contracts in the winter, low
sag will indicate a high tension, and as a result of this
contraction, the line may snap. Sag is inversely
proportional to the tension of the line, and is given by
the formula below.
This will result in increase in sag and decrease in
conductor tension [M. Muhr, S. Pack, R. Schwarz, S.
Jaufer, 2006]
 Age: Conductor sag over time may increase
due to the effects of strand settling in and
metallurgical creep. A higher tension may be
used when the conductor is first erected to
allow for “settling in of conductor strands
and for subsequent metallurgical creep of
the conductor material
 WIND: A wind load on the conductor will
increase the apparent weight of the
conductor resulting in an in increase in
tension. The increase in tension will increase
the cable length due to elastic stretch by an
amount given by given by:
ΔL = (To − T) / EA
(ii)
To = the initial tension in Newton
T = the final tension
E = the coefficient of elasticity
A = the cross section of the conductor in
meters.
For high tensions, the sag should be small. For low
tensions, the sag should be high.
Clearances must also be
observed when stringing a line.
The normal clearances for
overhead lines are shown in the
table below. [Sag & Tension
Data Table, NESC, IEEE 1990]
Voltage Level
less than 66kV
66kV to 110kV
110kV to 165kV
greater than 165kV
Clearance to Ground
20 feet (6.1m)
21feet (6.4m)
22feet (6.7m)
23feet (7.0m)
EFFECT OF SAG IN TRANSMISSION LINE
It Reduces Excessive Tension: While erecting an
overhead line, it is very important that the conductors
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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(4) 627-630 (ISSN: 2141-7016)
This increase in resultant load will result in
an effective sag in an inclined direction with
both horizontal and vertical components.
 Pole movement: Any movement of
pole tops due to stay relaxation etc will
have the effect of introducing additional
length into the span
 Ice: Ice build up on the conductor will
increase the apparent diameter and
weight of the conductor. This is not an
issue in Queensland however the same
approach can be used for calculating
loads and sags if bird diverters are
installed along a span
 .Conductor load per unit length, span
 Conductor tension
 Level of supporting towers.
Another important factor is the required safety
clearance of conductors. Safety clearances are of the
following;
1. Vertical Clearance: Distance between conductors
and ground.
2. Horizontal Clearances: Distance between
conductors and distance between conductor and
structure [ Reason J., 1995].
How Sagging Line Mitigator (SLiM) Works
As high temperature increases conductor length and
its sag, SLiM changes its geometry (by contracting)
to decrease line length. As conductor temperature
returns to normal, SLiM returns (by expanding) to its
original shape. The device with no motors or
electronic controls remains always ready to respond
to the next conductor high temperature excursion
[Hardware to Prevent Sagging of Transmission Lines.
California Energy commission, fact sheet, 2004].
Tests at the Pacific Gas and Electric Company
facilities in July 2002 showed that SLiM reduces the
conductor sag significantly to eliminate sag
problems. The temperature increase on the
transmission lines was 130°F, from 80° to 210°F. The
test transmission line reduced the sag by 3 feet 8
inches when compared to the control transmission
line.
CONCLUSION
It has been known that sag on conductors increases
the length of conductors used for transmission. Hence
there is the need to increase tension by fixing the
conductors on pole properly. This will also eliminate
possible accident that may arise as a result of sag as
living things including human-being can accidentally
touch line conductors.
How to Prevent Excessive Sagging of
Transmission Line
Transmission lines that sag under heavy use and high
temperatures are the bane of transmission line
operators [Electric Transmission week, 2004,
publisher of power daily] these lines are not only
limited in the amount of power they can carry, but
sagging lines run a higher risk of contacting trees and
shorting out. A sagging transmission line in Ohio is
believed to be the event that triggered last year’s
blackout.
REFERENCES
Doug Leenhouts, eHow contributor, how to
determine the Sag in Transmission lines Electric
Transmission week, Monday 2 August 2004, from
the publisher of power daily, New product takes
active approach to limiting line sag
NEWS RELEASE California Energy commission,
fact sheet, October 1, 2003. Hardware to Prevent
Sagging of Transmission Lines.
Up until now, utilities have had two basic approaches
to dealing with line sag. They could either:
 Re-engineer the line,
 monitor it to assure that sag limits are not
breached
Re-engineering, this typically involves
 Reducing the distance among the
transmission towers because the more the
space between transmission towers, the
more the transmission line will sag.
 Raising tower heights and/or re-conducting
address the problem but is an expensive
solution when compared with monitoring it
to assure that sag limits are not breached.
Reason
J.,
1995,
Transmission
Line
Uprating/Dynamic Rating, Electrical World, August
1995, pp 22-25.
Sag & Tension Data Table, NESC, IEEE 1990
Seppa T O, Adams Jr. H W, Douglass D A, Coad N,
Edris A, Olivier P, Thrash Jr. F R. 1998, Use of Online Tension Monitoring for Real-time Thermal
Ratings, Ice Loads, and Other Environmental Effects,
CIGRE Meeting 1998, pp 1-5, Paper No. 22-105.
Seppa T O, 1992, A Practical Approach for
Increasing the Thermal Capabilities of Transmission
Lines, IEEE/PES Summer Meeting, pp 1536-1542.
Monitoring is a passive approach that manages the
problem of sag but doesn’t address the utility’s need
to maximize power flows.
Seppa T O, 1995, Accurate Ampacity Determination:
Temperature-Sag Model for Operational Real-time
Ratings, IEEE Transactions on power delivery, pp
1460-1470, V 10 No. 3.
Material Integrity Solutions Inc. of Berkeley has
developed another approach. The company developed
a device called “sagging line mitigator,” or SLiM,
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Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(4) 627-630 (ISSN: 2141-7016)
Seppa T. O., Cromer, E., Whitlatch, W.F., 1992,
Summer Thermal Capabilities of Transmission Lines
in Northern California based on a Comprehensive
Study on Wind Conditions, IEEE Power Delivery.
Stephen R., 1998, Real Time Monitoring – revised
document for final comment 18 December
1998,Convenor Cigre Panel SC22, WG12.
Transmission Line Reference Book, 345kV and
Above, Second Edition, EPRI Report EL-2500,
EPRI, Palo Alto, CA, 1982, pp. 415-417
Wook M B, Choi M, Deb A K, 1997, Line-Rating
System Boosts Economical Energy Transfer, IEEE
Computer Applications in Power, pp 36-39.
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