COORDINATION OF GENERATORS IN PARALLEL

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PE (always learning)

PE (always learning)

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Professional Engineer
Hey everyone,

I am needing some help with coordinating generators in parallel. It's kind of hard to explain this system without showing you exactly what it looks like so I've attached the information for reference (one line diagram and TCC).

Basically, I have (2) 1000 kW generators that feed into a generator cabinet that then feeds out to three separate services. Each of these services have 4000 amp mains. This is all at 480, 3 phase, 4 wire. The generator breakers for each 1000 kW generator have 1600 amp breakers. My question is: how do I coordinate these 1600 amp generator breakers with everything downstream?

Basically, I've tried to make sure that the 1600 amp breakers coordinate with the largest feeder breakers in each of the three services, but I know they will never coordinate with the 4000 amp mains in the generator cabinet and on the main panels. How should the 4000 amp mains be set for the generator cabinet and on each main panel? Also how should I realistically set the 1600 amp mains for the generator breakers? Currently I have their instantaneous portions turned off to coordinate, but that might not be the best option.

I've attached the SKM one line diagram and a TCC with my attempt at coordination of the breakers from the generator 3 down to MDP2 (see attached one line). The long time on the 4000 amp breakers was set down to 2000 amps, which I'm thinking is probably not the right idea.

Can someone take a look at this and let me know what you think? I would play someone to help walk me through this coordination process.

Again, see attached for reference.
 

Attachments

  • TIME CURRENT CURVE.pdf
    167.6 KB · Views: 10
  • SKM ONELINE DIAGRAM - ATTEMPT 7.pdf
    74.8 KB · Views: 9
I changed the gen cabinet back to 4000 amps on the long time portion and also back to 4000 amps on the main for panel MDP2, but tried to coordinate in the instantaneous region, see attached for attempt #2.

Regards
 

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  • TIME CURRENT CURVE - ATTEMPT #2.pdf
    167.5 KB · Views: 1
My apologies, ignore all previous attachments and use the ones below for reference. The other ones did not show the proper available fault current.
 

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  • SKM ONELINE DIAGRAM - LATEST.pdf
    79.8 KB · Views: 6
  • TIME CURRENT CURVE - LATEST.pdf
    168.7 KB · Views: 5
Of course (2) 1250kVA gens only have a FLA of 3000A combined, so even if both generators are running you will never be able carry 4000A.

I would coordinate the 1600A breakers with the generator performance curves (Overload curve – located in the upper 1 or 2 decades, Short circuit withstand capability point – located in the middle decade, Decrement curve – located in the bottom 3 decades) and forget about coordination with the 4000A CB.

SKM has some application info on their website you can checkout https://www.skm.com/applicationguides15.html
 
1. Molded case or insulated-case breakers in series will, in general, not fully coordinate due to the instantaneous trip elements. It may be possible, but not always. Since the generators produce fairly limited fault current, you might be able to do it by setting the instantaneous trip elements high enough that they never operate.

2. If the two generators are operating in parallel, the downstream breakers will see more fault current than either of the generator breakers. This generally works to your advantage. The worst-case for coordination is probably operating on only one generator. But just looking at the TCC does not tell the whole story since the devices may see different values of current for the same fault.
 
I find it helpful to look at the single line using the branch fault datablock. It will show how much fault current each generator is seeing for faults downstream of the paralleling board.
 
For fault coordination, the best method is to graphically work out the trip times for each protective device under different conditions taking into account the different value of fault current that they see. If you can coordinate when running on only one generator, then it should coordinate on two generators.
 
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