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Generator SLG fault current vs. ISC 3p

Generator SLG fault current vs. ISC 3p

Generator SLG fault current vs. ISC 3p

(OP)
I recieved a cut sheet from a gen manufacuter and told him to comply with ANSI C50.13 which will limit the ground fault current Isc SLG to a value less than or equal to the 3 phase fault of the generator. My comment was to increase the zero sequence impedance 3x's the value of my positive sequence. My direct axis subtransient is .13pu, X2=.19pu and my X0=.02pu

2500kVA

480/277

His response: That standard only applies to generators 10MVA and larger. Not necessary for a generator of this size.
 

RE: Generator SLG fault current vs. ISC 3p

Is there a question?

RE: Generator SLG fault current vs. ISC 3p

(OP)
Yeah there is a question (hidden a little though - my apologies)
Since that standard applies only to machines 10MVA (c50.13) larger,
Q1- So is there a standard for lower machines?

Q2 - Also do I need to provide more zero sequence impedance to account for the low zero sequence impedance on the generator?

Q3 - What does the industry do? I see a lot of generators that have a really low zero sequence and I don't see any impedances added to them?

RE: Generator SLG fault current vs. ISC 3p

If you need a four-wire system (with 277 V single-phase loads), the generator neutral must be solidly grounded.  So the machine will have to deal with whatever forces it may experience during a ground fault.  

I would ask the generator manufacturer to verify that the generator is adequately braced for a bolted line-to-ground fault at the generator terminals.  

For medium-voltage generators, they are usually grounded via some type of impedance.   

David Castor
www.cvoes.com

RE: Generator SLG fault current vs. ISC 3p

(OP)
Thanks.  I appreciate the feedback.

RE: Generator SLG fault current vs. ISC 3p

And most 480V generators are braced for SLG faults.

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Generator SLG fault current vs. ISC 3p

(OP)
Terrific.

Thanks a lot for all your responses

RE: Generator SLG fault current vs. ISC 3p

For generators less than 10 MVA, use NEMA MG 1. In the scope section (Article 1) of the ANSI C50.13 has also this recommendation.
- MS

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