design considerations for transmission lines 1

[psozot]

can any one tell me about electrical design considerations of transmission lines.Is there any thumb rule for calculating sag for transmission lines and for errecting supporting poles? details will be appreciated

 

[sbr]

This is a pretty general question on design, so .... In general the design is based on the size of wire used, distance between poles, code requirements/clearance considerations (higher voltage=more clearance), total line length, wood or steel structures, lower voltage/cable/telephone underbuilds, etc.

As you can see there is a lot to take into consideration. Even with more details from your end I don't know how much help I can provide as far as rules of thumb.

 

[tgott]

Check out Rural Utilities Systems' Design guide 1724E.200 at

http://www.usda.gov/rus/electric/bulletins.htm

 

[Gmoney]

psozot,

For shorter spans, sag can be approximated by parabola: SAG = (Conductor Weight x (Span Length^2))/(8 * Tension). Sag is actually a catenary curve whose calculation involves hyperbolic trig. Keep in mind that tension can vary dramatically with conductor temperature and ice.

REA (or RUS) has published guides for line design which are probably available on their website,

http://www.usda.gov/rus

; The National Electrical Safety Code (NESC, IEEE C2) has loading districts defined, as well as other helpful data. These are for USA; I'm not familiar with requirements elsewhere.

 

[redtrumpet]

Aluminum Association Electrical Conductors Handbook (still available, search the net for Aluminum Association) has some useful engineering design data, including stringing charts, ruling spans, sag-span parabola, etc.

Also some utilities publish their design standards, with stringing charts for various sags and ruling spans already determined. They also have anchoring and pole setting standard details. However, these are usually more often found for distribution than transmission.

 

[nareshks]

The curve of cable supported at two poles is defined as catenary (hyperbolic cosine). This is an even function and hence there is a symmetry around vertical line passing through the bottom most point (SAG). From the catenary equations the SAG and length of the cable can be calculated.

The equation of catenary is:
Y = (T/w)cosh(wX/T) + C
Where
T = Horizontal Component of tension at the supporting point of cable.
w = weight per unit of the cable

Length of the catenary (cable) between two points 'a' & 'b' (Span length = b - a) is :
L = (T/w)[ sinh(wb/T) - sinh(wa/T) ]

 

[jbartos]

Suggestions:
1. Reference:
C.L. Wadhwa "Electrical Power Systems," John Wiley & Sons, 1991
Chapter 7 "Mechanical Design of Transmission Lines"
2. The sag has already some posting in this Forum. Try Advanced Search for quick results.
3. Sag is defined as a difference between an arbitrary point on the line P(x,y) and the lowest transmission line point H. The maximum sag d is
d=(L**2)/(8 x c)
where
L is the distance between 2 poles
c = To/W
To is the tension at the lowest point, i.e. point H
W is the weight per unit length of the wire

 

[nareshks]

Hi psozot,
Actually I am alos working upon making a software for designing of Aerial Optical Fiber Cables. Yesterday I was browsing net and came across the follwing site which has explaind in brief yet precisely how to determine sag, length of cable, tension etc.

http://www.du.edu/~jcalvert/math/catenary.htm

Hope this helps. Keep us updated.

 

[jbartos]

Comment: The simple equations for calculating sags are actually approximations. It is necessary to keep in mind (not stated in the URL reference of the last posting) that the approximate equations for the sag are valid for small values of the sag only. If sag is as depicted in the reference URL picture, the result for the sag would be very inaccurate.