Electrical Power Systems

Analysis of Sag and Tension
15.1 INTRODUCTION
Analysis of Sag and Tension 373
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Analysis of sag and tension of conductor is an important consideration in overhead transmission
as well as distribution line design. The continuity and quality of electric service depend largely
on whether the conductors have been properly installed. Thus, a design engineer must determine
in advance the amount of sag and tension to be given to the conductor at a given temperature,
maximum wind, and possible ice loading. In order to specify the tension to be used in stringing
the line conductors, the values of sag and tension in summer and winter conditions must be
known. Excessive tension may cause mechanical failure of the conductor itself because conductors
tension contributes to the mechanical load on structures at angles in the line and at dead ends.
The main factors in the design and stringing of conductors on the supports are:
1. Conductor load per unit length.
2. Conductor tension.
3. Span, that is, distance between supports.
4. Temperature.
or determining the conductor load, the factors that need to be considered are:
1. Weight of conductor itself
2. Weight of ice or snow clinging to conductor.
3. Wind blowing against conductor.
The maximum effective weight of the conductor is the vector sum of the vertical weight of
the conductor and horizontal wind pressure. It is extremely important to include the most
adverse condition. rom the design point of view, it is considered that the wind is blowing at
right angles to the line and to act against the projected area of the conductor, which includes the
projected area of ice or snow that may be clinging to it.
rom the practical point of view, economic design dictates the following:
1. Sag of conductor should be minimum to refrain from extra pole height.
2. Sufficient clearance above ground level.
3. To avoid providing excessive horizontal spacing between conductors to prevent them
swinging together in midspan.
Sag of the conductor decreases because its tension pulls the conductor up. But at the same
time, tension elongates the conductor from elastic stretching which tends to relieve tension and
sag increases.