Line Impedance Calcs
Line Impedance Calcs
(OP)
When calculating line impedances, what temperature is typically used for the sag calculation? Sag is often listed at the highest temperature rating of the line, but this is rarely the case under normal loading. Is there a standard industry practice when modeling lines or is it usually up to the engineer to choose a temperature that will be a good representation of the line during normal loading or under fault clearing times?






RE: Line Impedance Calcs
If you're accounting for the presence of the neutral on underbuilt distribution, you may want to consider a lower operating temperature for that neutral than you do for you line phase conductors.
RE: Line Impedance Calcs
RE: Line Impedance Calcs
The rise and fall of the mid-span point can be impressive, but the overall length doesn't change by much at all.
Do you calculate your fault currents with all possible combinations of generation running? We certainly don't, there's a bit of error there right off the bat; fortunately most of "source impedance" is transformers and other lines and not generation unless one is very close to generation. I've found that doing impedance calcs assuming straight line conductors produces results that are "good enough". If you truly want to improve the match between modeled results and actual fault results, I'd concentrate of improving your representation of the zero sequence environment rather than trying to add one more significant digit to the impedance values. The text book approach to zeros sequence modeling may produce a Z0 value as much as 120% of what it actually is, as measured by fault analysis. If you can make a 20% improvement or tweak a fraction of a per cent, which is better? Obviously that 20% improvement is based on a number of factors, some or many of which won't apply in your case; but personally I'd try to get a better ρearth value than 100Ωm long before going after sag effects.
But, pick a temperature value that makes sense in your conditions, document what you're doing and march forth. Would you use the same temperature for ACSS as you do for ACSR and AAC? Yes or no are probably equally valid answers depending on your circumstances. Do you want the maximum impedance for worst case voltage drop during high power flows, or do you want the minimum impedance for worst case fault currents?
RE: Line Impedance Calcs
https://www.omicronenergy.com/en/products/transmis...
While above you are measuring the R at ambient, keep in mind that any variations are taken care of via step distance with normal graded zones. R will vary no matter where you calculate it as conductor temps never stay constant. Personally Id be more concerned with fault resistance then conductor R variance.
RE: Line Impedance Calcs
RE: Line Impedance Calcs
I assume your questions is for traditional distance protection. For traveling wave type fault locating algorithms, instead of regular circuit miles, actual parabolic conductor length is used.
RE: Line Impedance Calcs
First line we did was just shy of 100 miles long and the good old oil breaker had enough pole scatter that the reflected wave from the first pole closed arrived before the second pole closed. Took a bit of help to figure that out.