Utility fault calculation 1

[calabala]

Hello,
I am working on a project and I can not get the utility fault information or the X/R ratio.
The utility is connected to a 27000/600 delta/ wye transformer, 2500 KVA with %Z of 5 and X/R ratio of 10.

Is there any way to calculate the 3-phase utility fault same as the X/R. If not, is there a way to estimate it based on some tables.

 

[dpc]

If you are just concerned with the secondary (600 V) side, a conservative approximation can be made by considering the transformer primary connected to an infinite bus.

In this case, it is about 48,100 A, symmetrical at 600 V for a three-phase bolted fault. This does not include any motor contributions from the secondary loads.

The value you use for the fault current depends on the purpose of the calculation, to some extent.

 

[alehman]

Yes. That would be appropriate for short circuit device evaluation. Not always a good assumption for arc flash calculations however.

 

[calabala]

Thx DPC and Alehman,
By the way, why this utility fault estimation won't be that accurate for arc flash study?
As for high voltage part (>15kV), Ralph Lee equation is used for incident energy calculation (is that becase incident energy is a fcn of fault current)?

 

[tem1234]

calabala,

Yes, incident energy is fct of fault current, anf of clearing time too. So if you estimate utility fault current higher than it is, the clearing time may be faster at this value (if you're not in the instantaneous range).

The incident energy is often less in this situation then if you take the real utility fault with a longer clearing time.

Bye the way, utility should give you this info if you ask them.

 

[wbd]

This thread, thread238-158408, is very similar to this discussion.