4. INTRODUCTION
In India and another foreign countries have referred the Overhead
Transmission Lines for maintaining the Power Supply to the distributed
areas.
Sag is a fundamental content of the Overhead Lines. Also, overhead
transmission lines has very economical and has better safety rather than
Underground Cables.
5. ABOUT SAG
While talking about Overhead Lines, it is very important that conductors
are under safe tension. If, conductors are too much stretched between
the supports, stress may reach in unsafe values and certain cases
conductor may break due to excessive tension.
In order to permit the safe tension in conductors, they are not fully
stretched but have a Dip or Sag.
SAG: The difference in level between points of supports and the lowest
point of conductor called SAG.
6. ABOUT SAG (Cont..)
It shows that conductors suspended between two
equal supports A & B. It allows the Dip (Sag).
The lowest point on conductor is O & Sag is S. It
has Parabolic Shape.
When the tension at any point on the
conductor acts tangentially. Thus,
tension To at lowest point O acts
horizontally
7. DIFFERENT TYPES OF SAG
There are basically 2 types of Sag are available:
1. When supports are at Equal levels,
2. When supports are at Unequal levels.
Low conductor tension and minimum sag are not possible. Because, low
Sag means a tight wire and high tension, whereas low Tension means a
loose wire and increased Sag.
8. CALCULATION OF SAG
1. When Supports are Equal:
Let, L = length of the span,
w = weight per unit length of the conductor,
T = tension in the conductor.
9. CALCULATION OF SAG (Cont..)
2. When Supports are Unequal: L = Span of the conductor,
h = difference in height level between two supports,
x1 = distance of support at the lower level point A from O,
x2 = distance of support at the upper level point B from O,
T = tension of the conductor,
w = weight per unit length of the conductor.
10. CALCULATION OF SAG (Cont..)
2. When Supports are Unequal:
So, having calculated the value of x1 and x2, we can
easily find out the value of sag S1 and sagS2.
11. ICE AND WIND LOADING
Above formula is only applicable for the constant air and normal temperature.
But, for apart of this we should use this calculations:
Total weight of the conductor per unit length is,
12. ICE AND WIND LOADING (Cont..)
Let us assume, w = weight of the conductor per unit length,
wi= weight of ice per unit length,
wi= density of ice × volume of ice per unit length
ww = force of wind per unit length
ww = wind pressure per unit area × projected area per unit length
13. ICE AND WIND LOADING (Cont..)
If the Slant Sag is appear in the Overhead transmission Line is denoted by
the,
So the verticalsag,
14. EFFECT OF ICE & WIND LOADING
Effect as follows:
The weight per unit length of the conductor is changed when wind blows at a certain force on the
conductor and ice accumulate around the conductor.
Wind force acts on a conductors to change the conductor self weight per unit length horizontally in
direction of the air flow.
Ice loading acts on the conductors to change the conductor self weight per unit length vertically in
direction of the air flow. Resulting weight is all then downward directions.
15. WHY SAG IN TRANSMISSION LINES??
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, it has effects of Ice, Wind or Temperature on it.
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.
So, Sag is inversely proportional to Tension.
STRINGING CHARGE: It is the proportional ratio of the Temperature and Depth of
Sag.
16. SAG FOR DIFFERENT RANGE OF LINES
Less than 66kV 20 feet (6.1m)
66kV to 110kV 21 feet (6.4m)
110kV to 165kV 22 feet (6.7m)
greater than 165kV 23 feet (7.0m)
17. ADVANTAGE & DISADVANTAGE OF
SAG
ADVANTAGES OFSAG:
Sag determines the value of safe working tension and the minimum clearance of
the conductor with respect to ground.
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.
DISADVANTAGE OF SAG:
If the Sag is too small then the Tension between two supports will increases so, it
leads to break the conductors.
18. CONCLUSION
By refer this theory, we are come on that point to understand and get
importance about the SAG in Overhead Transmission Lines. It is one of
the necessary part among that all.
If, any of the lines doesn’t have the SAG in it, it may be break out or may
be falls out with the poles in any time.
