NCEES Power Problem #530

[pepwr09]

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:

 

[knight1fox3]

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:

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?

 

[cruzy]

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

 

[pepwr09]

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.

 

[knight1fox3]

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!