Generator Fault Current

[kellytshort]

OK so I am paralleling 5 2500kW Generators and I am installing a NGR to limit my Isc 3p=Isc SLG (heard that was an ANSI standard) and at my genertors I've some 147kA of fault current at each Generator during a fault condition at the Generator itself. I looked at the TCC for a 200kAIC 4000AF circuit breaker and coordinated that with the damage curve of the generator and it appears to be fine. So a generator can take some 150kA of fault current for 1 second or so (reference attached docs). thoughts...

 

[rcwilson]

If the fault is in the generator, there won't be much left except melted copper after one second. If the generator is feeding the fault, it might last for one second. You may need to check your generator decrement curves against the circuit breaker trip curves. It is possible that the generator voltage will collapse and the fault current will reduce below the trip value of the generator.

The final fault current from a 2500 kVA generator is usually less than full load current. Initial fault current is determiend by the subtransient reactance which is usuallly about 15-20%, meaning fault currents 5-6 times full load. Final fault current is determined by the system impedance and the generator's synchronous reactance which is in the 200-250% range. Xd=200% means the final fault current will be half rated full load current. How fast the current moves between these two values is shown on generator decrement curves. If the current drops too fast, it can go below the breaker's pick up value and the breaker will stop trying to trip so the fault never clears. The curve shape is a function of the generator design and the type of exciter and voltage regulator used.

 

[skeshesh]

OK so I am paralleling 5 2500kW Generators and I am installing a NGR to limit my Isc 3p=Isc SLG (heard that was an ANSI standard) and at my genertors I've some 147kA of fault current at each Generator during a fault condition at the Generator itself. I looked at the TCC for a 200kAIC 4000AF circuit breaker and coordinated that with the damage curve of the generator and it appears to be fine. So a generator can take some 150kA of fault current for 1 second or so (reference attached docs). thoughts...

Sounds like you're on the right track. Bob's comments sound fine as well, but it seems to me he stated the general form of what you already have done, which is to find the specific manufact. damage curves and the specific TCC for the ocpd you will be using. Are you using any software package to do this study? SKM, ETAP and others have pretty extensive libraries nowadays so it'll make your life easier to get your company to buy a package like that (not to mention the reports look slick ). What's the synchronization scheme and protection further downstream if you don't mind me asking?