Mechanical Design of Lines

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CONTENTS
> Sag in Transmissison Line : 
> Span between Supports :
> Tension in Conductor : 
> Formula for sag calculation :
> Factors affecting sag :
> Effect of ice loading on sag :
> Effect of wind pressure on sag  :

What is Sag in Transmission Line ?

Sag in conductor : When the conductors are supported at the towers or poles, they are pulled and connected between supports but soon conductors will sag or dip under its own weight.
 
Maximum sag is essential in determining the ground clearance of the conductor. The sag should be minimum in order to reduce the conductor material and to avoid extra height of pole.


Span : The distance between adjacent supporting towers is called span. 

Tension in conductor : 
To avoid the mechanical faiilure of the conductor the tension in the conductor should be low. The tension in conductor depends upon conductor weight, effect of temperature, ice and wind loading.  

So sag and tension in the conductor should be so adjusted that tension in the conductor should be within safe limits.


How to calculate sag in Transmission Line?

Span having equal level of supports ( i.e. towers are at equal height ) is called level span and when the span has unequal levels of supports ( i.e. towers are at unequal height ), then it is called as  unequal level span.  
 
Formula for sag calculation : 
Sag and tension of the conductor is calculated for two conditions 

(a) When towers are at equal height. 



From above formula we know that
 Factor affecting sag are :

Conductor weight :  Sag of the conductor is directly proportional to its weight. The weight of the conductors is increased due to ice loading. 

Span : Sag is directly proportional to the square of the span length. Longer span gives more sag. 

Tension :  The sag is inversely proportional to the tension in the conductor. Higher tension increases the stress in the insulators and supporting structures.   

Wind : It increases sag in the inclined direction. 

Temperature : The sag is directly proportional to temperature i.e. sag reduced at low temperature and increases at higher temperature. 

 
(b) When towers are at unequal height.


Now using X1 and X2 in S1and S2 we can easily find sag. (Note - X1+ X2 = l )


Effect of ice loading on sag :

When conductor is used in the conditions where ice falling is common then conductor is coated by ice.  The weight of ice acts vertically downwards, with the weight of conductor. So, weight of ice per unit length is given by,  


Wi  = Density of ice × Volume of ice per unit length

So total weight of conductor with ice 
Wt = Wi + W (weight of the conductor)

 
Effect of wInd Pressure :

Conductor weight is also affected by wind pressure. The wind force per unit length is given by 
Ww = wind pressure per unit area × projected area per unit length



Effect on sag due to ice and wind loading:

We have learned that weight of ( ice+ conductor ) acts vertically downward and weight of wind pressure acts horizontally,

Then net weight of the conductor and its inclined angle will be 



Now sag in the conductor will given by

and vertical sag will given be

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