Looking at an old arc flash study done for a project I'm working on, I notice they derived the X/R ratio for the utility transformers:

"An X/R ratio of 7.29092 was used to model the 2000 kVA utility service
transformer designated UTIL XFMR MAIN BLDG obtained from IEEE Std.
C37.010-1999. The transformer nameplate impedance is 5.76%."

So I notice C37.01-1999 is not a free-to-access document. Anyone know about these formulae? Not trying to get something for free that we should be paying for...is this something any of you have used? The formulas or the document?

I'm not familiar with that IEEE document nor am I sure what your voltages are or if it matters but I used the %Z and X/R function on SKM and clicked on Typical X/R and got the same exact value as in your old study. My assumption here is that SKM may be using whatever is in that IEEE doc for this function.

The good news is a transformer X/R ratio is usually not a deal breaker. It can be determined with transformer test reports which I assume you don't have (you would be in a large minority if you did) so here is a bit of additional information.

IEEE C57 (Transformers) standards list "typical" X/R ratios and ranges of impedances for transformers.

Do NOT us a typical impedance as this has a direct impact on the short circuit current. This should be from the name plate.
However, "typical" X/R ratios are generally OK and software often defaults to these values as bbaumer points out.

The X/R ratio is used for the math. It is the tangent of the impedance angle of the transformer which is used to add impedances vectorially (including affect of angles) Whether the X/R is 6 (80.5 degrees), 7 (81.9 degrees), 8 (82.9 degrees) etc. should have a minimal impact on the results. Of course, it's better to use values that provide conservative answers but again, it shouldn't vary all that much.

Here is an article that I wrote about 20 years ago. Although it is old, the theory has not changed
https://brainfiller.com/electrical-safety-codes-and-standards/x-r-ratio/

Hope it helps!

I don't recall a formula for X/R, only a graph (IEEE red book -1993, page 184) which is reproduced in the IEEE red book, buff book, etc. Also, the IEEE standard referenced is for the application of HV circuit breakers. I wish IEEE wouldn't duplicate info in their standards...just makes things confusing and an admin nightmare to manage.

Mike

The X/R has to do with the asymmetry in the fault current that leads to the large first peak. ANSI C37 requires evaluating this for circuit breaker adequacy as the breakers are tested with a specific X/R (asymmetry) based on ANSI standards. If the X/R is higher than tested (depends on the edition of the standard) then adjustments must be made to the calculated fault current.

Yes, there was a tremendous amount of overlap between various standards. Worse, it would take years to revise them and they would become out of synch with each other. IEEE began an initiative around 20 years ago - I was at the initial meeting - the goal was to create a separate standard for each topic and eliminate the overlap. This became known as the 3000 series. I have an article about this below.

https://brainfiller.com/2017/06/19/2018-nfpa-70e-update-whats-new-whats-changed/#more-4201

This is one reason why IEEE 1584 does not directly address short circuit and coordination studies as some would like. As soon as we go down that path, then we are back to having duplicate information.

Thank you for the articles! It's good to know I'm not leaving out a huge factor in my modeling.

Thanks for you comment and your posts. Loads of articles and videos at Brainfiller.com and adding more all the time.
Best wishes!