Treatment of emergency standby generators in short circuit current calculation studies

[BHENG]

Hi all
I was wondering if there is an explicit statement in our professional literature (NEC, IEEE, IEC) about treatment of emergency standby generators (e.g. in AUTO mode) when calculating short circuit currents.
Are we allowed to ignore contribution from those machines?
Cheers
BH

 

[waross]

Standby sets are normally offline and have no effect on normal short circuit currents.
When online, the SC current from the generator is typically so much less than the grid short circuit current that reliable coordination may be an issue. The standby generator may not supply enough current to trip breakers on instantaneous. While waiting for an inverse time trip rather than an instantaneous trip, voltage collapse may occur with more issues.
Often the issue may be not enough SC current to ensure reliable operation of protective devices with settings based on grid SC currents.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

I agree with Bill if the purpose of the short circuit calculations is to determine adequacy of equipment ratings. However, if you are doing arc-flash calculations, you cannot ignore the generator. It is necessary to calculate arc-flash levels for both operating conditions and then take the worst-case of the two.

 

[BHENG]

Thanks for quick replies...I see an installation where the primary backup generator is being backed-up (with an identical machine)...both on AUTO mode and several seconds of time delay between loss of utility and loss of primary generator.
What is the right fault current calculation method? If the main controller fails - it is theoretically (and practically) possible to have all three sources feeding the load bus!
Re the arc flash calculations...what do you suggest?
BH

 

[dpc]

Unless the two generators have synchronizing capability, only one can be connected to your loads at one time.

 

[waross]

Closing in a second source without synchronizing gear is theoretically possible but practically impossible. You have about a maximum window of 60 degrees out of 360 for an unsynchronized closure to succeed. Plus or minus 30 Degrees out of 360 is a 5 out of 6 chance of failure. I have seen successful closures at 30 degree error but that may well be outside the envelope of many installations. 15 degrees plus or minus is more than many operators would be comfortable with. Closing in a second source takes the odds to 1 in 36. Theoretically possible but practically impossible is a good description.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

How are they switched? Typically with an ATS it is physically impossible to have both sources at the same time. With paralleling gear, on the other hand, the whole point is paralleling the sources. For equipment ratings, assuming an ATS, I'd completely ignore the generator. If there's paralleling gear, you have to consider both in parallel, and the fault current, particularly for SLG faults, of both paralleled can be significantly higher than the sum of each alone. For arc flash you have to look at each and every case. Often the generator alone, with much lower fault currents, will produce a far higher hazard due to much longer clearing times.

 

[BHENG]

The whole scheme utilizes paralleling gear (circuit breakers and some simple relaying scheme - i.e. no ATS).
My concern is that no robust PLC control is in place and long control wiring running hundreds of yards between various relay panels is so unreliable (in long term), consequently an undesirable switching arrangement is quite a possibility (especially taking into account a very large number of machines connected together). Could anyone suggest a reputable specialist consultant to help re this issue?

 

[dpc]

Could anyone suggest a reputable specialist consultant to help re this issue?

If you told us your location perhaps you would get a better recommendation. This is an international forum.

 

[waross]

Try e3ach parallel setup as a stand alone unit. One generator and associated synchronizing and connecting gear. Then send a single signal to each unit when it is to go online. Keep all the controls other than a two wire external signal local to the paralleling contactor or switch. Then a fault on the external circuit will commend the set to make a proper and orderly connection top the bus. No harm done. Similarly an open in the external control circuit will result in a fail to connect condition. Again no harm done.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[living2learn]

I have had many installations where the generators contributed more fault current than the utility. It consisted of very large generator plants that paralleld upwards of (5) engines onto a common bus. The zero sequence current available from generators seems to be pretty low compared to the sub transiant and had two different instances when the SLG fault was over 200kAIC.

Off topic, but when I utilize closed transition transfer switches which do parallel the sources I always add the fault contribution from both sources.

Food for thought. Always do the calcs...