NEMA MG-1 Clarification

[gvasiliou]

Generator datasheets refers that manufacturing of generator is according to NEMA MG 1-32.

Part 32 of NEMA MG-1, section 32.13, NEMA Refers:

32.13 SHORT-CIRCUIT REQUIREMENTS
A synchronous generator shall be capable of withstanding, without damage, a 30-second, three-phase
short circuit at its terminals when operating at rated kVA and power factor, at 5-percent over-voltage, with
fixed excitation. The generator shall also be capable of withstanding, without damage, at its terminals
any other short circuit of 30 seconds or less provided:
a. The machine phase currents under fault conditions are such that the negative-phase-sequence
current, (I2), expressed in per unit of stator current at rated kVA, and the duration of the fault in
seconds, t, are limited to values which give an integrated product, (I2)2t, equal to or less than
1. 40 for salient-pole machines
2. 30 for air-cooled cylindrical rotor machines
b. The maximum phase current is limited by external means to a value which does not exceed the
maximum phase current obtained from the three-phase fault.

Can somebody explain this .... with more details...?
I admit is a little confusing term since 3ph short circuit capability of generators is described, but I(2) is involved, and as far as i know, I(2) is involved only in Single Phase to ground short circuit....
Thanks.

 

[magoo2]

You might get a better answer if you posted this under the forum for electric motors and generators.

My take on this is that, if you're dealing with a three phase fault, you won't have any negative sequence current developed. I'm not sure why they even mentioned it.

 

[xnuke]

The generator shall also be capable of withstanding, without damage, at its terminals
any other short circuit of 30 seconds or less provided:

This says "any other short circuit," which I interpret to mean any unbalanced fault since they've already discussed balanced faults, and all unbalanced faults produce negative sequence current.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[odlanor]

I would like to include another information on this subject from IEC 60034-1-2010 Short-circuit current for synchronous machines

9.8 Short-circuit current for synchronous machines
Unless otherwise specified, the peak value of the short-circuit current for synchronous
machines, including turbine-type machines not covered by IEC 60034-3, in the case of short circuit
on all phases during operation at rated voltage, shall not exceed 15 times the peak value or 21
times the r.m.s. value of the rated current.

Verification may be carried out by calculation or by means of a test at a voltage of 0,5 times the
rated voltage or above.