Generator - fault current for downstream devices

[mrbj]

Hi,
please help resolve the following discussion.
What value of rms sym fault current do you use when attempting to determine the settings of a downstream protective device (ie an mccb)when the supply is from a generator?.
Do you use either the sub-transient, the transient or the steady state conditions.
ie.
600kVA generator at 415/240Volt approx 835Amp
X"d at 0.10 giving Irms = 8350
X'd at 0.15 giving Irms = 5567
Steady state sus short circuit current at approx 2600A.
I err towards the transient, my collegue towards the steady state.

 

[RRaghunath]

Subtransient for short circuit (electromagnetic) release and transient for overload / overcurrent release.
This works well in relay coordination studies.

 

[wbd]

Is this the same as the 1/2 cycle and 5 cycle numbers?

 

[rbulsara]

wbd:

I would say yes...may be rraghunath can pitch in again.

 

[RalphChristie]

Normally three periods of concern, first the subtransient period, then the transient period, and finally a steady state period.
The subtransient period is short and has a time constant of 0.015 to 0.05 sec (0.9 - 3 cycles at 60hz, 0.75 - 2.5 seconds at 50hz)
The transient time constant have a typical range of 0.3 to 0.7 seconds (18 - 42 cycles at 60hz, 15 - 35 cycles at 50hz)

Short circuit calculations are often made for 3 different delay times, depending on the need. A first-cycle calculation is made for the withstand capability of equipment and fast interrupting times of fuses and low-voltage devices. In this case, the subtransient current is of concern. For high-voltage breakers that interrupt in 1.5 to 8 cycles, using an impedance that is a modified subtransient impedance would be reasonable. In fault calculations for time delay relaying of 0.3 to 0.5 seconds, use the transient current.

This is from A practical guide to short-circuit calculations by Conrad St. Pierre, a highly recommended book.