# PU analysis, #530 or similar



## Jonneyo (Mar 20, 2010)

Question: In problem 530, the Zpu of the utility = Sbase/Fault Duty.

Why does this equation equal a Zpu? I also don't understand what is meant by fault duty. In this problem Zpu = 1MVA/40MVA = .025 How does this equal an impedance?


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## yellowjacket03 (Mar 20, 2010)

See the following thread:

http://engineerboards.com/index.php?showtopic=11332

There are several methods to calculate fault currents. One is the Ohmic method and another is the Fault MVA method. The NCEES sample questions uses the Ohmic method. I prefer the Fault MVA Method. Its much quicker. I reference a great paper in the thread posted above. Scroll down to the bottom of the thread.



Jonneyo said:


> Question: In problem 530, the Zpu of the utility = Sbase/Fault Duty.
> Why does this equation equal a Zpu? I also don't understand what is meant by fault duty. In this problem Zpu = 1MVA/40MVA = .025 How does this equal an impedance?


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## Jonneyo (Mar 25, 2010)

Thanks yellowjacket.

I found the article and I like the MVA method for its simplicity. However, I still feel like I need to have an understanding for the PU method for similar problems that provide data on PU basis. The test makers seem to like this and I think I'm finally getting a grasp of it.


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## Art (Mar 25, 2010)

what you want is Zf pu = Zf / Zb, Z under a bolted fault...

we assume an infinite buss and Vb remains the same, no droop...

Sf = Vb^2 / Zf or Zf = Vb^2 / Sf and

Sb = Vb^2 / Zb or Zb = Vb^2 / Sb

so Zf / Zb = (Vb^2 / Sf) / (Vb^2 / Sb) = Sb / Sf as is illustrated in the problem...

and it makes empirical sense:

what limits power flow? Z

so as the allowable power flow under fault decreases, it's intuitive that Z must increase

Z = 1/40 = 0.025 (Z pu)

if only 20 MVA = fault duty

Z = 1/20 = 0.05

or fault duty = 1 MVA

Z = 1/1 = 1

so as Zf increases the power flow under fault must decrease...


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## nmh0408 (Mar 31, 2010)

Jonneyo said:


> Thanks yellowjacket.
> I found the article and I like the MVA method for its simplicity. However, I still feel like I need to have an understanding for the PU method for similar problems that provide data on PU basis. The test makers seem to like this and I think I'm finally getting a grasp of it.



The MVA method is more simple, but how do you use it when the questions doesn't provide all MVAs, like NCEES question number 513, where the utility MVA is missing?


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