Sag and Tension

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. (ii) Final 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/deadend/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. Conditions for Sag Tension Calculations Condition Minimum Temperature No Wind Minimum Temperature 36% Wind Everyday Temperature No Wind Everyday Temperature Full Wind Everyday Temperature 75% Wind Max Temperature No Wind - EW Max Temperature No Wind Conductor

Temperature in Deg C

Wind Pressure in Kgs (Ex: Zone-3-132kV Tower)

0 0 32 32 32 53

Conductor 0.0 48.0 0.0 131.0 99.0 0.0

Earth wire 0.0 59.0 0.0 162.0 122.0 0.0

75

0.0

0.0

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.

Sag Tension Output from TowerCad Condition (Min. Temperature-0 degC. Maximum Temperature for Earth Wire - 53 Deg. C. Maximum Temperature for Conductor - 75 deg C)

Minimum Temperature No Wind Minimum Temperature 36% Wind Everyday Temperature No Wind Everyday Temperature 100% Wind Everyday Temperature 75% Wind Max Temperature No Wind - EW Max Temperature No Wind - Conductor

Maximum Sag & Tension

Conductor Tension in Sag in Kgs mm.

2884 3397 2285 4485 3857 1998 1766 4485

4322 3670 5456 2780 3233 6241 7060 7060

Earth Wire Tension in Kgs

Sag

in mm.

1415 1738 1194 2472 2123 1076

3890 3167 4608 2226 2593 5117

2472

5117

Sag & Tension  The Sag Curve Template based on the above Calculation.

Sag & Tension  Observe the Conductor Technical Particulars and Design span considered in the Sag template curve used by your surveyor.

Sag & Tension  Observe the Sag, Ground Clearance & Sag error consideration in the Sag template curve used by your surveyor.

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 • What is a ruling span?

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