Generator short circuit

[AusLee]

Hi,

I want to calculate the short circuit for 3 generators in parallel. The purpose is to specify the circuit breakers. Gensets are 2500 kVA each.

By hand:
1. Current per phase @ 230V I = 3623 A.
2. Worst case x"d = 0.11.
3. Max short circuit current (symmetrical) = I/x"d = 33000 (more or less).
4. (3) gensets in parallel=> Isc max = 33 kA x 3 = 100 kA

I think too high.

By software (DOC Win from ABB):
1. For one generator, Isc = 10 kVA (a bit less actually).
2. For 3 gensets, Isc = 24 kA.

Which one is correct you think?

 

[ruggedscot]

When you short a genset you are limited by what the AVR can whack out - this can drop the output and result in a reduced shortcircuit current leading to increased time to clear the fault from the system. protection is always an issue with this sort of set up as you spec for mains then you have to spec for generator.....

 

[AusLee]

Ok, i understand that a lower Isc will take longer time to trip. Terasaki have "specially calibrated I2t curves for generators" in the electronic relay on the air circuit breakers (AGR-31B Type "S").

So what will be the sc on the synchronized busbar?

 

[genhead]

Your hand calculation looks about right for the generators themselves.
The software solution looks way too low, but what have you got in between the generators and the bus?

 

[glendale]

A quick modelling of your generators (using SKM power tools) produces 23kA per gen and a maximum of 63.7kA on the busbar with all three connected.

 

[ijl]

I agree with genhead, the hand calculation looks correct. Has glendale perhaps taken the 230V as the phase-phase voltage, and included the "c-factor" of 1.1 in the calculations?

Auslee, is it possible to have a look at your DocWin-file (the *.ile-file)?

ijl at

http://pp.kpnet.fi/ijl

 

[lz5pl]

AusLee, when modeling your (?) setup by DOC using default generator: turbogenerator 2500 kVA, 400 V phase-to-phase voltage, xd"=12.5 %, I receive 35 kA/generator, what is close to your hand calculations.


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It may be like this in theory and practice, but in real life it is completely different.
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