Estimating X/R Ratio

[Grollor33]

Hi all,

I first want to apologize because I can't imagine that this question hasn't been asked...but nothing turns up in my search for X/R in the key-word...so here goes:


I find that in estimating X/R ratios, I always end up using a graph of typical X/R versus machine kVA from a GE tech paper (or ANSI C37.010). Now those aren't too shabby of resources, but then my estimates are not derived directly from my given info.

Say I was given kW, kVA, rated voltage, stator & field resistance, subtransient, transient, and synchronous d and q reactances, zero seq and neg seq reactance plus all associated time constants with a generator's open circuit and short circuit tests...

I'd guess that coming up with X/R of the source would be a slam dunk...but I can't come up with a number for it even in the ballpark of the ANSI reference - but I have a software package that hits it nearly on the nose (by some type of magic, I presume)...

I'm embarrassed by this...what am I missing (other than an intuitive grasp on the subject)?

 

[dpc]

The generator reactances used in SC calcs are really rough approximations of a very dynamic process.

Your software most likely just has a lookup table based on the ANSI chart you mentioned.

Conrad St. Pierre has some discussion in the Appendix of his book indicating that X/R can be approximated as 2pi times frequency times ta3 (short circuit dc time constant)

You may want to grab a copy of his book "A Practical Guide to Short Circuit Calculations" for more info.



David Castor

http://www.cvoes.com

 

[DanDel]

You could also ask the software manufacturer for their calculation algorithm.

 

[wolf39]

Grollor33:

What you mean with X/R ratio? Synchronous machines have reactances like

synchronous reactance Xd and Xq

transient reactance X'd and X'q

subtransient reactance X"d and X"q

and quite a few more.

Regards

Wolf

http://www.hydropower-consult.com