×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Generator Fault Current

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

The short circuit bracing for a generator pertains to its ability to withstand the short circuit current it produces for an external fault.  If the fault is inside one of the generators, it's already failed.  So these are two different issues.  

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

(OP)
I have no Line to neutral loads and the Circuit breaker I am using has a fuseless current limiting feature by cutler hammer.  They do stand behind there rating and yes you are correct that is how they get the 200kAIC rating.  

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

Quote:

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?

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

Is the 31kA a value from the OEM? Question how it behaves over a duration of a few cycles, because the excitation system can have a big influence especially if it is a bus-fed exciter. Also consider that a fault close in to a generator typically causes problems for a circuit breaker because of the very high X/R ratio and the long time to first current zero. C-H standing behind the 200kA breaking capacity is one thing, but make sure the person answering the questions recognises that a generator breaker is a different animal to a distribution breaker.

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.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Generator Fault Current

(OP)
From C-H guy

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

When you apply a breaker like this you must verify that the breaker will be operated within the parameters and limitations of the UL testing that its rating is based on.  

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

Clearly you are on the ragged edge of the circuit breaker ratings. You may want to consider limiting the fault current with some added impedance in the generator circuits.

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources