# NCEES Problem 530



## Machiavelli999 (Oct 20, 2010)

It's a pretty bad sign that I am having trouble with a problem this close to the exam, but...

I have no idea how they get system thevanen impedance of 0.025 pu, which they use to find the fault current.

I got a completely different answer.

SCD = 40 MVA

Voltage = 12.47kV

Zth = 12.47kV^2 / 40MVA = 3.88 ohms

Then I divide it by 40 to bring it down to a base of 1,000 kVA and I get Zth = 0.09719 pu.

How do they get Zth = 0.025 pu???


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## Machiavelli999 (Oct 20, 2010)

Hmmm...wish I could just delete posts. I got it. The 3.88 ohms needs to be divided by the base impedance since it's an actual ohm value and not a pu value. Hope I don't have these brain farts on the exam.


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## cableguy (Oct 20, 2010)

Don't feel bad, the more problems you work, the more brain farts you have.  Mine for today was when I transferred a 3 ohm impedance to the high side of a 3:1 transformer. Circle the answer "9", keep going.

I shouldn't miss ones like that... Whoops! Shows me I can't be overconfident, nor let my guard down.


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## cruzy (Oct 23, 2010)

Machiavelli999 said:


> Hmmm...wish I could just delete posts. I got it. The 3.88 ohms needs to be divided by the base impedance since it's an actual ohm value and not a pu value. Hope I don't have these brain farts on the exam.


So your Zbase = Z of the 40 MVA system?? Meaning your Zpu of the system is 1pu, which at a 1000kVA base = 1*(1000kVA/40,000kVA) = 0.025. Which makes the System Zpu = 1/40MVA. I think I saw the formula somewhere in my studies saying Zpu = 1pu(Voltage)/Isc,pu = 1/MVAsc,pu.

IMO, there are so many way of solving problems, that's why we're all getting these brain farts.


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