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Estimating X/R Ratio

Estimating X/R Ratio

Estimating X/R Ratio

(OP)
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)?

 

RE: Estimating X/R Ratio

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
www.cvoes.com

RE: Estimating X/R Ratio

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

RE: Estimating X/R Ratio


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
www.hydropower-consult.com

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