3 phase fault current Q

[iwire]

a 3 phase fault occurs at the output of a delta-wye 115:13.8kW, 30/40/50MVA transformer with z=0.01 + j0.08 pu on a 50MVA base. The feeder has r=0.1pu and x=0.1 pu with a 240A rating.

Determine the fault current.

 

[iahim]

I get about 9200A with MVA method, and about 9600 if I solve in pu. Am I close? I have assumed that the feeder is on the primary side of the transformer.

 

[iwire]

the answer is 9617A.

how do you do it? The solution on the book is confusing. both method

 

[iahim]

With pu:

1. Covert the line impedance to the same base as the transformer. I used 115kV and 50MVA as base. So Zline=0.148<45

2. Calculate the total impedance 0.217<58.16

3. Calculate the short circuit current 1/total impedance = 4.6<-58.16

4. Calculate the base current = 251A

5. Calculate primary sc current = 1154.82A

6. The above current is on the primary side, so convert to secondary = 9623A

 

[iahim]

With MVA:

1. Covert the feeder impedance to ohms. R=27.66 ohms and X=27.66 ohms, So Z = 39.12 ohms

2. Calculate Z for transformer (in pu) = 0.08 pu

3. Use MVA to solve: line contribution is 115^2/39.12 = 338MVA and transformer 50/0.08=625MVA. So total is 219.37MVA

4. Calculate the current at 13.8kV = 9382A

 

[Spark-E]

Shouldn't the 2 methods result in the same answer? Why is there a difference?

MVA method can be found here: www.arcadvisor.com/pdf/ShortCircuitABC.pdf

(via thread http://engineerboards.com/index.php?showtopic=17353)

It helped me a lot with these problems.

 

[iahim]

I have made some approximations in the calculations, so that could explain the difference in the two results. It's also possible that I've made some error (you can double check me). I doubt on the exam they will have answer choices so close that you get a different choice depending on the method you used. Most likely the other choices will be obtained by making common errors, and they will be way off.

 

[nasir]

Would you please calculate how you get R and X values...??

 

[iwire]

Would you please calculate how you get R and X values...??

Zpu = Z/Zbase

Z = Zbase(Zpu)

Zpu = 1 pu

Zbase = 115k /(240)(1.73)