# NCEES Power Problem #530



## pepwr09 (Oct 28, 2010)

Pretty much wrapped up the NCEES power practice problems except for number 530. Might be a simple solution but I can't see it in their solution. Also having a hard time picturing this problem as well. It calls for a 1,000kVA, 12.47kV-480Y/277V transformer with 4% impedance which feeds a 480V bus. The fault duty of the 12.47kV system is 40MVA. Assuming the transformer and the 12.47kV system have the same X/R ratio, you are asked to find the 3-phase short circuit current at the 480V bus. In their solution I am not sure how they got the 0.025pu impedance when calculating the short circuit current.

Any info would be appreciated.

Thanks a bunch.

Good luck to all.

lease:


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## knight1fox3 (Oct 28, 2010)

pepwr09 said:


> Pretty much wrapped up the NCEES power practice problems except for number 530. Might be a simple solution but I can't see it in their solution. Also having a hard time picturing this problem as well. It calls for a 1,000kVA, 12.47kV-480Y/277V transformer with 4% impedance which feeds a 480V bus. The fault duty of the 12.47kV system is 40MVA. Assuming the transformer and the 12.47kV system have the same X/R ratio, you are asked to find the 3-phase short circuit current at the 480V bus. In their solution I am not sure how they got the 0.025pu impedance when calculating the short circuit current.
> Any info would be appreciated.
> 
> Thanks a bunch.
> ...


Have a look at this thread. Basically you have convert the transformer S (1MVA) to the same base as the system S (40MVA). 1 / 40 = 0.025p-u Does that help?


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## cruzy (Oct 28, 2010)

If you assume a 1 MVA base (per the transformer rating), then:

1. Zpu of system is (12.47^2/40)/ (12.47^2/1) = 0.025

2. Ztotal = 0.025 + 0.04 = 0.065

3. 1pu volts/ 0.065 = 15.38 pu amps

4. Ibase = 1,000 kVA/ (sqrt 3 * 0.480) = 1202.8 amps

5. Isc = 15.38pu * 1202.8 amps = 18500 Amps


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## pepwr09 (Oct 28, 2010)

Thank you cruzy and knight1fox3.....makes sense......time to wrap things up and get ready for tomorrow. Not sure if it is me but I found the morning NCEES problems easier than the afternoon. Since I am taking the exam for the first time not sure if that is the case or not.

Good luck.


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## knight1fox3 (Oct 28, 2010)

pepwr09 said:


> Thank you cruzy and knight1fox3.....makes sense......time to wrap things up and get ready for tomorrow. Not sure if it is me but I found the morning NCEES problems easier than the afternoon. Since I am taking the exam for the first time not sure if that is the case or not.
> Good luck.


Have a look at this thread. There is some brief discussion on what people thought of the April 2010 exam and which section was _thought_ to be more difficult. To be honest, it really is subjective. Good luck!


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