Problem 530 (NCEES)

[LJEngineer]

MVA Method

40MVA in series with 25 MVA (1MVA/.04)

40 X 25 / (40+25) = 15.38 MVA

Fault is on the 480v bus

15.38MVA/(sqrt(3) * 480e-6MV) = 18,500A

Wow, thank you so much! This was definitely easy to solve. 

 

[yaoyaodes]

what is 12.47-kv-480Y/277-V mean?

Does it mean that the primary is in the range 12.47-480 kv, Y connection? And the secondary is 277-V?

 

[no_concentrate]

It means Primary is 12.47kV ( Delta) and secondary is 480V (Y connection) 480V Line voltage and 277 is Line to Neutral Voltage.

 

[David_EE]

what is 12.47-kv-480Y/277-V mean?

Does it mean that the primary is in the range 12.47-480 kv, Y connection? And the secondary is 277-V?

12.47 kV is the distribution line voltage (High side voltage). It could be 3 wire Delta 12.47 kV (no ground wire connection) or 4 wire WYE 7.2 kV (12.47/sqrt(3)=7.2) line-ground, 12.47 kV line-line). It depends on how you connect the transformer.

The low side winding of each single phase transformer is 277V line to ground. WYE configuration is three phases line to ground. If you measure between two phases 277V@ 0 deg and 277V @120 deg, you will read 480 V line to line. Or 277*sqrt(3) = 480V.

So each single phase transformer could be: 12.47kV/277V (Delta HS/ WYE LS) or 7.2kV/277V (WYE HS/WYE LS).
There is no 480V winding, it is a product of the phase angles.

Note, it could be a 3 phase transformer, which includes all three single phase sets of windings inside one transformer. In practice an overhead transformer bank (on the pole) is usually 3 individual transformers. And underground pad mount (on the ground) transformers are generally 3 phase transformers.

 

[salmanshu322]

we can use MVA Method, 40MVA in series with 25 MVA (1MVA/.04)
and 40 X 25 / (40+25) = 15.38 MVA
we find out that Fault is on the 480v bus
15.38MVA/(sqrt(3) * 480e-6MV) = 18,500A

 

[Nunnelee]

Very Neat!!!!!

I didn't know about the following formula: ":Transmission per unit is equal to MVA_Base_3_Phase/Fault_Duty_3_Phase " but your approach makes perfectly sense to me. I would be curious to know how they derived this formula. Can you email, post or refer me to a book talking about Fault_Duty_3_Phase? Not much time left for April exam.

Thanks!!!!

I am working through this problem with the resource pdf provided by NCEES. It doesn't provide that information... I wish it was a bit more inclusive at times.