Typical Generator X over R ratio Clarification in IEEE 399 1

Wfg42438 · Jan 20, 2021

Hello Everyone,

I just wanted to see if anyone out there may be able to clarify a question I have about Fig 1-36 in IEEE 399-1997.

In this Figure, they specify typical X/R ratios for Generators, and for small generators, they have "Low", Med" and "High" curves.

Does anyone know what these individual curves represent?

dpc · Jan 20, 2021

They are just providing an approximate range for smaller machines. I wouldn't read too much into it.

cuky2000 · Jan 21, 2021

The curve shown was developed using statistical data from rotating machines manufacturers in north America in the 50´s and 60´s.

In absence of better information, this values can be used in the early stage of the project as unconfirmed assumption to be verified after test

Wfg42438 · Jan 21, 2021

Thanks for the background info on the curves.

When using these as references how would I know which of the curves is applicable to me?

What exactly tells me to use the low, medium or high curve?

davidbeach · Jan 21, 2021

Use the worst one for what ever you’re considering. Like using the low end of an impedance range for fault calculations and the high end for load flow studies.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

Wfg42438 · Jan 21, 2021

I see what you mean we take the conservative approach based on the "High" curve.

Seems like its a drastic difference in each case, I wonder if "high" value would overly conservative.

Thank you for the input!

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Typical Generator X over R ratio Clarification in IEEE 399 1

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