# NCEES Power Practice Exam #540



## BebeshKing PE (Jan 26, 2020)

On problem 540, is it supposed to be just the MVA fault contribution of the generator G1 alone the one being asked? And should be just 834/0.23 =3626MVA? Or am I missing something? Could someone please explain?

thank you for your help!


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## LyceeFruit PE (Jan 27, 2020)

Because the fault is on the 230kV bus, the transformer is part of the fault contribution. 

If the fault was on the 22kV "bus" (between the gen and the xfmr) your thought would be correct.


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## Orchid PE (Jan 27, 2020)

It's also because the generator will only supply the amount of current required for the fault.

834 / 0.23 = 3626 Will essentially give the the total fault current that is possible for the generator to supply. However, since the fault is on the other side of the transformer, the fault current is somewhat "limited." So it just needs the _contribution_, not the total fault current that _can_ be produced by the generator.


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## DLD PE (Jan 27, 2020)

Good question and good answers.  For these types of problems, I try to remember to do 3 things:

1.  Put an "X" (if it's not already there) at the described fault location.

2.  Underline the type of fault (single phase, 3-phase, etc.)

3.  Keep in mind what's being asked (it's asking for the short circuit contribution of the generator), so I'll circle "G1".

Under pressure, you never know when you might be in the middle of a problem and you end up forgetting what you're trying to solve.  Little notes like this can keep you on track.


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## Zach Stone P.E. (Jan 28, 2020)

BebeshKing said:


> On problem 540, is it supposed to be just the MVA fault contribution of the generator G1 alone the one being asked? And should be just 834/0.23 =3626MVA? Or am I missing something? Could someone please explain?
> 
> thank you for your help!


Hi @BebeshKing, the problem is essentially asking for the amount of fault power that originates from the generator that makes it through the transformer all the way to the fault bus. Transformers are known as "current limiting devices" because during fault conditions there is only so much current (or fault power) that they will "let through" to the other side, even if the transformer is being fed from an infinite bus.

Because the transformer is in series with the generator, you'll have to take both into consideration when solving this question.


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## BebeshKing PE (Jan 29, 2020)

Zach Stone said:


> Hi @BebeshKing, the problem is essentially asking for the amount of fault power that originates from the generator that makes it through the transformer all the way to the fault bus. Transformers are known as "current limiting devices" because during fault conditions there is only so much current (or fault power) that they will "let through" to the other side, even if the transformer is being fed from an infinite bus.
> 
> Because the transformer is in series with the generator, you'll have to take both into consideration when solving this question.


Thank you @Zach Stone, P.E.. Now it's clear.


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## BebeshKing PE (Jul 17, 2020)

BebeshKing said:


> On problem 540, is it supposed to be just the MVA fault contribution of the generator G1 alone the one being asked? And should be just 834/0.23 =3626MVA? Or am I missing something? Could someone please explain?
> 
> thank you for your help!
> 
> ...


 quick question for this guys, what if the problem ask for the short circuit MVA contribution of  transformer T1,? will it be just 933/0.15= 6220MVA?


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## jd5191 (Dec 23, 2020)

Somewhat of a nitpicky question on problem 540, should they have provided the transient reactance X'd for this problem? Would it have made more sense to either have Xd (steady-state) or X''d (subtransient)?


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## Dothracki PE (Dec 23, 2020)

@jd5191 Typically fault contribution is calculated with sub-transient reactance or transient reactance, whichever is given. If all three are given, the sub-transient is used.


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## lturner (Feb 28, 2021)

BebeshKing PE said:


> quick question for this guys, what if the problem ask for the short circuit MVA contribution of transformer T1,? will it be just 933/0.15= 6220MVA?


Wondering this myself. I think you are correct but am responding to bump this for others' input.


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