# NCEES 513



## tabiolite (Oct 10, 2010)

Question

For a 3-phase fault at location a, calculate the 12kv short circuit current is

Tline = j.0145/1000ft

T1 - 3-Phase, 60/12-kV, 7.5MVA Xfmr, with 7.5% impedance (inductive)

60kV

]

]________ T1 ______Tline _____ X____ to resistive load

]

]

Simple fault question

Can't figure out where I am going wrong when I choose 7.5MVA as my base.

The solution chooses 100 MVA as the Sbase.

Isc = I base * (1/Ztot)

Zbase = (sqrt(3)*12kV)^2/7.5 MVA = 57.6 ohms

Ztline = j15.321/57.6 = j.268pu

ZT1 = .075pu

Ibase = Sbase/(sqrt(3)*12kV) = 360A

Isc = 360/ (1/(.075+j.268) = 1294A

The answer should be 415A


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## DK PE (Oct 10, 2010)

tabiolite said:


> Question For a 3-phase fault at location a, calculate the 12kv short circuit current is
> 
> Tline = j.0145/1000ft
> 
> ...


Check your formula for Zbase in step 2, I used Z base = (base voltage kV L-L )^2/ base MVA 3-Ph = 19.2 ohms.

From there your work looks fine and I followed and got 412 Amps.


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## Flyer_PE (Oct 10, 2010)

DK PE said:


> Check your formula for Zbase in step 2, I used Z base = (base voltage kV L-L )^2/ base MVA 3-Ph = 19.2 ohms.
> From there your work looks fine and I followed and got 412 Amps.


I did the same thing and got 414 Amps. Probably just carried through more numbers on the calculator.

ZBase=VBasekV2/MVAbase=122/7.5 = 19.2 Ohms


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## Nucky (Oct 14, 2010)

Flyer_PE said:


> DK PE said:
> 
> 
> > Check your formula for Zbase in step 2, I used Z base = (base voltage kV L-L )^2/ base MVA 3-Ph = 19.2 ohms.
> ...


I am new to this board but just a comment on this (It has got me a few times). You have to be careful how you calculate base values and how you use base values. In this example, if you choose 7.5 MVA as the base you can either calculate Zbase as:

Zbase = (12 kV / sqrt 3) ^2 / (7.5 MVA / 3) OR

Zbase = (12 kV) ^2 / (7.5 MVA)

If you notice, in the first equation (sqrt 3) ^2 / 3 = 1, so you end up just getting the second equation. (tabiolite you did not divide 7.5 MVA by 3)

Overall with per-unit you just have to be careful and realize that you can take the per-unit value (voltage, current, power etc.) and convert it to back to either a single phase or three phase value depending on what base you multiply by and what value you are interested in finding. For example a voltage of 1 pu could either be 12 kV / sqrt 3 (phase voltage) or 12 kV (line voltage). A power of 1 pu could either be 7.5 MVA / 3 or 7.5 MVA.


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