Short Circuit Model Question

[Dumbo2929]

For those of you that do short circuit studies, I have the following. What has been your experience with the accepted convention for assuming motor contribution when specific motor data is unavailable?
Is it common to assume a value (say 25% of the main switchgear size) and put a lumped motor model at the main switchgear. This is more simplistic than a motor model at each panel, but for small systems may lead to similar results.
I always try to model large motors based on actual data, but for a planning study, that data is not present.
Thanks

 

[wareagle]

If you have a copy of the IEEE red book there is aome info available. I think they lump motors 50 hp and below. I am aware of some who neglect all but the large motors.

 

[cuky2000]

See Thread238-102304

 

[Dumbo2929]

I appreciate the response. I try to model all motors over 25hp and lump the smaller ones. That plus a conservative look at panel ratings usally works. I don't allow panels with 21999A fault current to have a 22kA rating for example. There are some that do however. 50% of switchgear size seems high for commercial buildings, since most load is lighting and receptacle. I would expect mechanical HVAC load to be closer to 25%. Any thoughts............

 

[dpc]

There are ANSI calculation methods that are recommended, but it is somewhat dependent on the breaker ratings you are evaluating. Low voltage breakers are evaluated a little differently than medium voltage.

But if you lump motors below 50 hp in one group and 50 hp and above in another group, this will comply with ANSI requirements, provided you apply the correct reactances to each group.

If you are doing short circuit studies, I would recommend purchasing Conrad St. Pierre's "A Practical Guide to Short Circuit Calculations".

 

[Dumbo2929]

Thank you for your help.

 

[cflatters]

I have done a number of studies using SKM (IEC909 and the voltage factor C1).

Take a realistic look at the motors which can operate at a any given time, ignore standby units and lumping smalller motors.

If using VSD's I tend to use Xd"=1 (thyristor commutation failure ocurrs resulting in negligible regen). Otherwise Xd" varies between approx 0.2 for larger motors and 0.167 for smaller units using DOL or assisted start.