X/R Calculation

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MyCleveland

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Cleveland, Ohio
Can anyone provide some feedback on this topic please.

Attached is a generic power layout with how I believe the cct simplifies from left-to-right.

First, is the simplification technique correct for what is shown ?

Second, I am assuming the SKMs and Easy Power programs have the X & R values for motors and transformers built into the program....from what DOCUMENT(s) (IEEE or ANSI) can I obtain these values ?
 

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ron

ron

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New York, 40.7514,-73.9925
I've never seen that simplification technique.

SKM and Easypower etc, all have typical values for X and R for motors and transformers, or x"d for gens etc in the library, but they can be overridden if the specific equipment data is known.
 
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jlee405

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The diagram looks kind of like the MVA method, where you can treat the series impedances like an equivalent parallel resistor and the parallel impedances like a series equivalent resistance.

Not sure which ANSI tables they are, maybe c57 or c37. But I use the GE whitepaper on short circuit calculations that has all the x/r ratios in it.
 
ron

ron

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Location
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Yes, several books have typical values.
I beleive IEEE [h=1]551-2006 - Recommended Practice for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems has some for example[/h]
 
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jlee405

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Location
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MyCleveland said:
Ron
"in the library" is what I am looking for.

jlee405
How are you accounting for the exact X/R of the power system at every point ?
Click to expand...

I'm not sure what you mean by that. Each electrical component (conductor, transformer, motor) has it's own X/R value that contributes to the whole system or whatever point you are faulting at. You also will need to enter in the X/R value from the utility source that is provided by the POCO (I typically use 8-12 if not given). I think of it like a thevinin/norton equivalent circuit.
 
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MyCleveland

Senior Member
Location
Cleveland, Ohio
jlee405 said:
I'm not sure what you mean by that. Each electrical component (conductor, transformer, motor) has it's own X/R value that contributes to the whole system or whatever point you are faulting at. You also will need to enter in the X/R value from the utility source that is provided by the POCO (I typically use 8-12 if not given). I think of it like a thevinin/norton equivalent circuit.
Click to expand...

How are manipulating X/R of components without knowing the actual X & R of each component ?
Understand about utility component, I just posted a sketch as a talking point.
Does the GE white paper have tables of actual X and R data for transformers and motors ?

Could you post a PDF of a simple one-line with method of attack ?
Utility (assumed value) serving bus 1
bus 1 with two branches (277/480Y)
Branch one feeds 10HP
Branch two feeds 30kVA trans (120/208Y)
Trans serves one 20HP motor
 
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jlee405

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Location
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Take a look at this website: http://arcadvisor.com/faq/motor-transformer-xr-data
Gives you some typical x/r values for different devices. I can't find that GE whitepaper, left it on a thumb drive somewhere.

Actual X/R is unknown until the device is built, that's why we use the ANSI tables for assumptions. Actual X/R values can be found on the nameplates of devices, and is why software like easypower let's you enter it in manually otherwise they use the ANSI values by default I believe.

X/R is another interpretation of power factor. I'm not sure what you are trying to accomplish, but I typically look at x/r values when dealing with asymmetrical fault currents and equipment duties.
 
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MyCleveland

Senior Member
Location
Cleveland, Ohio
I am trying to calculate the X/R at each point for the same reason...Assy. Component....
If we go back to my original sketch and add in a utility source...am I approaching the problem via my simplification indicated so I can come up with the X/R at each point.
 
J

jlee405

Member
Location
Seattle
If you're trying to do this by hand, there is a good example in the IEEE red book that does a similar type of calculation you are trying to do and they show the x/r tables in there. However, I haven't really read that portion in depth. If you are trying to calculate device duties, keep in mind there are x/r multiplying factors involved. When a device's tested x/r is smaller than the calculated x/r, the multiplying factors are used to give you sort of like an equivalent asymmetrical in symmetrical current for the device to see if it's underrated or not.
 
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