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Sag & Tension

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Sag & Tension
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Sag & Tension PRESENTATION PREPARED BY T.M MADANA SEKHAR M.Tech, MBA. Divisional Engineer / Designs APTRANSCO
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Page 1: Sag & Tension

Sag & Tension

PRESENTATION PREPARED

BY

T.M MADANA SEKHARM.Tech, M

BA.

Divisional Engineer / Designs

APTRANSCO

Page 2: Sag & 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.

Page 3: Sag & Tension

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.

Page 4: Sag & 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.

Page 5: Sag & Tension

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.

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Sag & TensionAs 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 0.0Max Temperature No Wind - Conductor

75 0.0 0.0

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

Conditions for Sag Tension CalculationsTemperature

in Deg CCondition

Page 15: Sag & Tension

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 7060Maximum Sag & Tension 4485 7060 2472 5117

Earth WireConductor Sag Tension Output from TowerCad

Page 16: Sag & Tension

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

Page 17: Sag & Tension

Sag & Tension Observe the Conductor Technical Particulars and Design span considered

in the Sag template curve used by your surveyor.

Page 18: Sag & Tension

Sag & Tension Observe the Sag, Ground Clearance & Sag error consideration in the Sag

template curve used by your surveyor.

Page 19: Sag & Tension

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.

Page 20: Sag & Tension

Sag & Tension

• What is a ruling span?

Page 21: Sag & Tension

Sag & TensionTension 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 .

Page 22: Sag & Tension

Sag & TensionAlmost 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.

Page 23: Sag & Tension

Sag & Tension The instruments used for measuring Tension & Sag

Electronic

Dynamometer are the preferred method for testing tensioning

Page 24: Sag & Tension

Sag & Tension The instruments used for measuring Tension & Sag

Mechanical Dynamometer

Page 25: Sag & Tension

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.

Page 26: Sag & Tension

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.

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Sag & TensionIn view of Short of time I am ending this. We can

meet with some more stuff after some time


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