Generator Transient & Subtransient Reactance

[genhead]

When calculating fault currents for a generator (400 volt 1250kva for example), I thought that we would use the subtransient reactance, X"d. However, I have just read a paper on short circuit currents that states "for salient pole alternators, subtransient reactance is negligible".
Can anyone explain what this might mean?

 

[davidbeach]

Never heard such a thing. For an accurate model of a salient pole machine you need both X_d'' and X_q''. Both are significant and X_q'' will be smaller than X_d''. The effective X'' is the average of the two. Is it possible that your quote is somehow taken out of context? If not, I wouldn't put much stock in the paper.

 

[ijl]

Negligible = very small = big effect on short circuit current?

 

[alehman]

I agree with David. "Negligible" has no meaning in that context.

 

[genhead]

Here's a little more information from the paper. I could post the whole document, but I don't know if that would be "allowable" as it would identify the author.
Anyway, this method would indicate short-circuit levels to be unrealistically low - or am I missing something?

 

[genhead]

I don't know what happened to the attachment. I'll try again.

 

[davidbeach]

Well, ain't that strange? That's the first calculation for generator fault I've ever seen that ignores subtransient and only uses the transient reactances.

 

[RRaghunath]

Some info from IEEE Std 141:

IEEE Std 141 (Red Book) gives typical Xd'' & Xd' values for Salient pole as well as Turbo-machines. Refer Annexure 4A. There is also a note there indicating that Xd' for Salient pole machines is not normally used in Short circuit calculations.
The Xd'' values for Turbo-machines and Salient pole machines are in the range 9-15% and 16-21% respectively. Corresponding Xd' values are 15-23% for Turbo-machines and 33% for Salient pole machines.

 

[RRaghunath]

There is one more issue, in this regard. That is, when we are calculating SC currents for the purpose of overcurrent relay coordination, it is customary to consider transient reactance of generator, irrespective of whether Salient pole or wound rotor one.
This is because the subtransient stage in generators would not last for more than 100ms (or 5cycles).
Hope you find it useful.

 

[davidbeach]

Equipment withstand ratings must be based on subtransient reactances. Anything that might have an instantaneous trip characteristic must also be evaluated based on subtransient reactances.