Generator Fault Current
Generator Fault Current
(OP)
I have (5) 2500kW generators in parallel at 480V and I heard it was an ANSI standard that the Isc 3P => Isc SLG during a fault so I am installing a NGR to limit the fault current on the ground. Now each of my generators produce 31kA 3p and 31kA SLG and when those five are in parallel I get some 150kA of fault current at one of my generators. So in SKM I coordinated the 200kAIC 4000AF circuit breaker with the damage curve of the Generators and it showed it being safe. Looking at the damage curve of the Gen's it starts out at around 1 second. So a Generator can take infinite amount of fault current during 1 second? My concern is these Generators are only braced for their available fault current and if one of them faults the other four will feed fault current to it damaging the Gen set? Thoughts..






RE: Generator Fault Current
RE: Generator Fault Current
RE: Generator Fault Current
But with all those generators in parallel on a common bus, massive damage will occur for a stator winding fault due to the extremely high fault current.
Additional concerns:
If your system is directly supplying any line-to-neutral single-phase loads, the grounding resistors must have an adequate continuous rating to deal with the heat that will be produced by the unbalance current.
The 4000A circuit breaker with 200 kA interrupting rating almost certainly has integral current-limiting fuses that give it that high interrupting rating. However, the generator fault contribution will be dynamic, decaying rapidly, so you may want to discuss this with the generator and breaker manufacturer to make sure they will stand behind the 200 kA rating in this application.
With 12,500 kW of generation, a medium voltage system might have been a better option - but without knowing more about your application, it's hard to say.
David Castor
www.cvoes.com
RE: Generator Fault Current
Boy oh boy if the client allowed medium voltage generators life sure would be good, but you can't win them all!
So if I have a fault in one of my Gen's that means it's done for anyways and has already failed beyond repair, so this point is irrelavant?
I think you are correct I need to get the Gen people and the circuit breaker people together, and have them sign off on this to a certain degree.
RE: Generator Fault Current
Sort of. The fault current rating and bracing pertain to external faults, not internal faults. An internal fault will generally always require some repair. The more fault current flowing into the fault, the more repair.
What C-H breaker is this?
David Castor
www.cvoes.com
RE: Generator Fault Current
Anecdotally I've heard of certain medium voltage breaker manufacturers who don't have a proper generator breaker in their range instead offer a distribution breaker with a few cycles worth of time delay in the trip circuit to ensure that the breaker holds in until the DC component dies down so the breaker interrupts at a current zero. If C-H are doing this - and I'm certainly not suggesting that they are - then ask yourself whether, with 200kA on the loose, you want a delay of any form in the trip string? I wouldn't - I'd rather have the right breaker for the job and pay a bit more for it.
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If we learn from our mistakes I'm getting a great education!
RE: Generator Fault Current
The MDSX is able to achieve the 200kAIC rating because it is a current limiting device and interrupts fault so quickly at very high fault currents. Attached are graphs showing the current limiting capabilities. The MDSX2000 is applicable for all 2000A and below while the MDSX4000 is for everything above 2000A. As explanation to the I peak Graph, taking in to account a power factor of 15% of the circuit (the 15% is UL standard for power breakers) there is a 2.3 factor between the peak and the symmetrical RMS AIC rating. Therefore, for the MDSX4000, at available fault current of 150k amps the peak let-through is 225k which you divide by 2.3 to get 98k rms let-through. If the device is fed by MDSX2000 breaker or below the rms let-through is about 85k amps. This is kind of confusing but hopefully I'm making sense. Let me know if you need further explanation.
RE: Generator Fault Current
RE: Generator Fault Current
Scotty has a very good point - you should get written verification from Cutler-Hammer that this breaker is suitable for use as a generator breaker in this application. I would want to use your generator and system data to calculate the expected max fault current and X/R ratio (power factor) that the breaker would be called upon to interrupt.
This is an extreme application for a current-limiting circuit breaker, due to the local generation.
David Castor
www.cvoes.com
RE: Generator Fault Current
Have you run the SKM Device Evaluation ?
Also be sure you are getting the correct generator data. The cut sheet you supplied is from the genset manufacturer's software. In a critical situation like this, it may be worthwhile to get specs directly from the generator manufacturer.
On the subject of limiting damage to the generator, getting the field shut down quickly may be more of a challenge than clearing the external source. There have been several papers published on this topic.
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.