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Utility and Generator Impedances


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#1 David Robison

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Posted 21 January 2008 - 08:15 AM

We are upgrading the fault calculations in the next release of Design Master Electrical to be based upon X and R, rather than just Z. In order to do this, it is necessary to specify X, R, and X/R ratios at various starting points. We are unsure how utility impedances and generator impedances are typically defined, particularly in areas of the country we do not do design work. We are hoping you can help us by sharing your experiences.

For the utility and for generators, what values are you given to define the impedence? Z with an X/R ratio? X and R? Max fault only? Something else?

#2 bjenks

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Posted 21 January 2008 - 09:34 AM

QUOTE(David Robison @ Jan 21 2008, 11:15 AM) <{POST_SNAPBACK}>
We are upgrading the fault calculations in the next release of Design Master Electrical to be based upon X and R, rather than just Z. In order to do this, it is necessary to specify X, R, and X/R ratios at various starting points. We are unsure how utility impedances and generator impedances are typically defined, particularly in areas of the country we do not do design work. We are hoping you can help us by sharing your experiences.

For the utility and for generators, what values are you given to define the impedence? Z with an X/R ratio? X and R? Max fault only? Something else?

Max fault current with the %Z and X/R of the service transformer found using IEEE curves if not supplied by utility.  With a open transition ATS and a 200kW or less generator it is very hard to get any fault currents from the manufacture.  There point is that the utility always has a higher fault current and thus it doens't matter unless the ATS is closed transition.

If I was you I would keep your fault currents very simple and just give the fault currents at each equipment point with contributions from the utility, motors >50hp combined and transformers.  assume standard NEMA X/R ratios based on the voltage and size of the equipment.  The only reason anyone should be using your fault current feature is for small/medium designs that are just looking for easy AHJ approval.  

For anything else the engineer should use SKM, EasyPower, or eTAP...  There is way too many options that need to be considered that I would think would be way past the scope of your program.

#3 donw

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Posted 21 January 2008 - 10:54 AM

For utilities, most cities around here require use of the utility's published available fault current for the given service size (or transformer size.) We are not even allowed to use the transformer-to-service conductor's impedance - just the published available fault current - right at the service entrance. Most engineers around here use the Bussmann Point-to-Point Method, but the city reviewers don't care, as long as you show what formulas you used.

For generators, you should use the published subtransient reactance, X"d of the generator in PU. Maximum Isc = (1.0/X"d) PU. Then use the generator's rated amps to get max Isc in Amps.

I'm interested in how you'll treat dry-type gen. purpose transformers. IEEE Std. 242-1986 has a table of impedance data (including suggested X/R ratio) for various sizes of transformers. Perhaps you could have DM start with these values and allow users to modify them.

#4 donw

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Posted 21 January 2008 - 11:04 AM

I do agree with bjenks. We really have not needed any more complex calculations than DM currently does for fault current. Except for motor contribution, which would be nice for large motors only.

#5 Guest_Mark Magee

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Posted 16 February 2008 - 10:36 AM

QUOTE(David Robison @ Jan 21 2008, 08:15 AM) <{POST_SNAPBACK}>
We are upgrading the fault calculations in the next release of Design Master Electrical to be based upon X and R, rather than just Z. In order to do this, it is necessary to specify X, R, and X/R ratios at various starting points. We are unsure how utility impedances and generator impedances are typically defined, particularly in areas of the country we do not do design work. We are hoping you can help us by sharing your experiences.

For the utility and for generators, what values are you given to define the impedence? Z with an X/R ratio? X and R? Max fault only? Something else?



It seems as though Z in per unit (%) and X/R ratio are a good way.  Using the X/R ratio you can see more readily if you are entering a good value or not.  Either way you can convert pretty easily.  As donw said, the IEEE publishes standard curves of X/R vs. kVA for machines and transformers.

Thanks for the fast response on this issue.  You guys make a great product that keeps getting better.




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