Generator Neutral Short Circuit Current

[spowell]

I'm working on a generator neutral bus design and I'm wondering what the short circuit current will be at the neutral. I can't find anything in IEEE on this subject. The spec for the isolated phase bus on the line side of the generator is 388kA Assym. Would the current on the neutral be 388kA/SQRT(3)?

Thanks for any help.
Steve

 

[rbulsara]

In case of a phase to neutral fault, the current in the neutral will be the same as phase current. Ask for or look up for that value for your project. Some has done the fault analysis.

Rafiq Bulsara

http://www.srengineersct.com

 

[cranky108]

Assuming the generator is impedance grounded, the fault current for a phase to ground fault will return through the neutral impedance, so if you know that impedance you can figgure the fault current. Usually this value is less than the normal current during production.

Rarely have I seen a neutral braced for phase to phase faults of three phase faults, which I attribuite to the thinking isolated phase bus can't fail anyway but phase to ground.

However, does anyone know what happens to the phases once they enter the GSU, or aux. transformer?

 

[spowell]

Thanks for the quick response on my question.

rbulsara: What do you mean by "phase to neutral fault".

One thing I need to add... This is a generator neutral tie with all three bushings tied together with one copper conductor. It is 35,000A forced cooled and has a neutral grounding cubicle connected to the neutral tie.

cranky108: When we terminate isolated phase bus to the the GSU, it is in a segregated box so the fault would also be pahse to ground.

 

[rbulsara]

Exactly what it reads. When would a fault current will flow through the neutral?

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

Phase to phase fault current will flow through the neutral bar which bonds the neutral ends of the three windings, as dictated by Kirchoff's Rule. In that respect the bonding bar must be braced for the maximum fault. The connection between neutral and earth only needs to be heavy enough for the maximum line-earth fault, which on a big set is normally less than 10A or so. Normally the smallest available MV cable is capable of carrying much larger current.


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