Overhead line Ratings 1

[Flashover]

Hi Folks,
I was wondering if anyone could answer the following:-
Flamingo 666.6 mcm overhead line?
1) i am trying to find curves of s/c capacity against tripping time
2) how do i de-rate this cable for warmer climates such as the middle east? also its 2 conductors per phase.

3) short circuit rating of the cable? what formula do i use

i have spent alot of time searching the web with little progress, any help greatly appreciated
many thanks
Flashover (young electrical engineer learning his trade

 

[wbd]

If you go to the web site for Southwire (

http://www.southwire.com)

they have a technical library. You can request a copy of their "Overhead Conductor Manual" which is only available hardcopy (free). This should have the info you need.
Also, the s/c capacity against tripping trip - are you looking for the Maximum Fault - Current Operate Curves? also known as cable melt curves - this should be available from the manufacturer.
In that Southwire book there is a whole chapter on Thermal Ratings of Bare Overhead Conductors, with example calc. Also included in that chapter is a section on Fault Current ratings toe determine the adequacy of conductor for s/c currents.

 

[Flashover]

Thanks i have done this already, unfortuanately this will take alot of time. your help is greatly appreciated
thanks
flashover

 

[redtrumpet]

1) and 3) Maximum Fault-Current Operating Limit for Bare Stranded ACSR conductor:

t = ((0.0862* m)/l)^2 where t = seconds, m = area in circular mils, and l = rms amperes

Upper temperature limit 645 degrees C, ambient temperature 40 degrees C. Time plotted is that required for a given rms fault current to bring aluminum strands to the threshold of melting (steel will be unaffected and will still supply the mechanical strength). It is assumed there is no heat loss in the conductor.

For 666 kcmil Flamingo, this time limit is .33 s @ 100 kA rms and 33 s @ 10 kA rms.

Reference: Aluminum Electrical Conductors Handbook published by the Aluminum Association

 

[redtrumpet]

2) If you observe the same allowable maximum temperature for the conductor; then the allowable temperature rise is less for a higher ambient temperature than a lower ambient temperature. So base your ampacity on the temperature rise. Admittedly, you need an ampacity graph to do this, which you may not have.

The change in ampacity for a given temperature rise, based on different ambient temperature starting points, is little changed over a considerable range of ambient temperature. Therefore, you should be safe in using standard temperature rise graphs, which are usually based on 40 degree C ambient, even if your ambient temperature is higher.

Of course, other factors have to be considered beside temperature rise, including sun, emissivity, wind conditions, etc.

 

[3b2]

Existing EHV transmission line? Find the longest span and determine the minimum line to ground clearance (courtesy of NESC). Determine the sag of the line at installation temperature or sag at existing temperature. Find normal load current and Isc at source and load terminals. If taps exist, then each needs to be determined. Line impedance Z1 and Z0 need to be known for each section. Impedences are calculated using info from Westinghouse T&D Reference Book(1964) or the Southwire Overhead Conductor Manual(Isc relationships are also available from this excellent source). Determine the worst case senario for line sag and temperature due to Isc at the longest span in the particular line section. U get to determine this from the very unfriendly catenary tension calcutations found in the above and other references. Using the worst case sag condition(temperature) U can determine how many seconds your particular Isc level allows.

If this is a new design, U have a bunch more fun things to figure out.

Summary, there is no easy way to determine what U want. This is a complex problem.

3b2

 

[Flashover]

thanks for your help guys, hopefully i can return the favour sometimes. i enjoy reading the posts on this website, its a mind of information
thanks Flashover