Sag & Tension
Sag and Tensions are the breath and heart-beat of Transmission Line Construction & Operation Engineers. Sag is defined as “The distance measured vertically
from a conductor to the straight line joining its two points of support.” In other words sag is usually the lowest point on a cable or wire between two points.
Tension is defined as “The longitudinal force exerted on a wire during installation”. Tension is the force pulling the wires at either end by what they are attached to or the weight of the cable itself.
Sag & Tension
Sag & Tension behaviour of conductors are very important to the practicing engineers because it rules the design, erection, maintenance and operation of any Transmission Line and its components.
There are two main types of tensions. (i) Initial Tension.
Sag & Tension
Initial Tension: Initial Tension is defined as “ The longitudinal tension in the conductor prior to the application of any external load”. In other words the characteristics of the conductor before time, temperature, weight etc. of the conductor stretch or shrink the wire.
Final Tension: Final Tension is defined as “The longitudinal tension in a conductor after it has been subjected for an appreciable period of the loading prescribed, in which it is situated, or equivalent loading, and the loading removed. In other words what the characteristics of the conductor are expected to be after time, temperature, weight and other factors have effected it.
Sag & Tension
Sag-tension Calculations???
Why Bother with Sag-Tension???
Sag determines electrical clearances, right-of-way
width (blowout), uplift (weights & strain), thermal rating.
Sag is a factor in electric & magnetic fields, Aeolian
vibration (H/w), ice galloping.
Tension determines structure deviation
angle/dead-end/broken wire loads.
Initial & Final Tension limits of Conductor, determine
conductor system safety factor, vibration, & structure cost.
Sag & Tension
As per Indian Standards the Sag & Tension Calculations will be carried out for the following conditions.
Conductor Earth wire
Minimum Temperature No Wind 0 0.0 0.0
Minimum Temperature 36% Wind 0 48.0 59.0
Everyday Temperature No Wind 32 0.0 0.0
Everyday Temperature Full Wind 32 131.0 162.0
Everyday Temperature 75% Wind 32 99.0 122.0
Max Temperature No Wind - EW 53 0.0 0.0
Max Temperature No Wind -
Conductor 75 0.0 0.0
Wind Pressure in Kgs (Ex: Zone-3-132kV Tower)
Conditions for Sag Tension Calculations
Temperature in Deg C
Sag & Tension
The Sag Tension Calculation results for the above conditions and for a
132kV Line in wind zone-3 with Panther ACSR Conductor and 7/3.15mm HTGS Earth wire for design Span of 320 Meters.
Condition
(Min. Temperature-0 degC. Maximum Temperature for Earth Wire - 53 Deg. C. Maximum Temperature for
Conductor - 75 deg C)
Tension in Kgs
Sag in
mm. Tension in Kgs Sag in mm.
Minimum Temperature No Wind 2884 4322 1415 3890
Minimum Temperature 36% Wind 3397 3670 1738 3167
Everyday Temperature No Wind 2285 5456 1194 4608
Everyday Temperature 100% Wind 4485 2780 2472 2226
Everyday Temperature 75% Wind 3857 3233 2123 2593
Max Temperature No Wind - EW 1998 6241 1076 5117
Max Temperature No Wind - Conductor 1766 7060
Maximum Sag & Tension 4485 7060 2472 5117
Earth Wire Conductor
Sag & Tension
Sag & Tension
Observe the Conductor Technical Particulars and Design span considered in
Sag & Tension
Observe the Sag, Ground Clearance & Sag error consideration in the Sag
Sag & Tension
In the Design Maximum Sag and Maximum
Tension will be considered.
Maximum Sag will give minimum clearance to ground. Due to over loading of lines and high ambient Temperatures in our state conductors are usually at high temp.
Maximum tension:- So that structures can be designed to withstand it.
Minimum sag - To check the structure uplift problems & H/w during “coldest month” to limit Aeolian vibrations.
Sag & Tension
Sag & Tension
Tension equalization between suspension spans allows use of the ruling span
Initial and final conditions occur at sagging and after high loads and multiple years
For large conductors, max tension is typically below 60% in order to limit wind vibration & uplift
Negative tensions (compression) in aluminium occur at high temperature for ACSR because of the 2:1 diff in thermal elongation between alum & steel .
Sag & Tension
Almost all composite conductors exhibit a “knee point” in the mechanical response.
At low temperature, thermal strain (or sag with increasing temperature) is the weighted average of the aluminum and core strain.
Above the knee point temperature, thermal sag is governed by the thermal elongation of the core .
Thermal strains cause changes in elastic strains. The computations are iterative and extremely tedious.
Sag & Tension
The instruments used for measuring Tension & Sag
Electronic Dynamometer are the preferred method for testing tensioning
Sag & Tension
The instruments used for measuring Tension & Sag
Mechanical Dynamometer
Sag & Tension
The instruments used for measuring Tension & Sag
Third wave return methd
A light rope tossed over the conductor near one end of a span and give it a good hard jerk down. At the same instant press the button on the watch to start it. You then feel for return pulses in the rope as the shock wave you created runs up and down the conductor. At the instant you feel the third or fifth return you stop the watch. Read the number on the appropriate scale on the face of the watch and you have your sag in feet. The scales are direct reading and no math is needed.
Sag & Tension
Finally - don’t forget to do the following.
1) Check whether Correct Sag Templates are used by Your Surveyor.
2) Check whether Correct Tower Spotting Data is used.
3) Use Stringing Charts and Tension & Sag measuring instruments without fail while doing Stringing.
4) Measure the conductor temperature instead of ambient temperature and find out the correct Sag & Tension from Stringing Chart for that temperature.
Sag & Tension
In view of Short of time I am ending this. We can meet with some more stuff after some time
