Calculating prospective fault currents in transformers

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Wire_Wizzard

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Dear mike your You tube page is awesome!

I however come from New Zealand and things are a wee bit different, but physics ain't To quote Admiral Montgomery Scott " you can nay change the laws of physics laddie" Star Trek. But true none the less. I have an exam preparation question and I have been hunting through all my old papers and the units on single phase and three phase transformers, but can't find a working formula or process to follow to help me answer the question and also have limited contact with my tutors.

Question: An 11kV /400v, 100kVA, three phase, delta-star transformer has a 5% impedance. When fully loaded a line current of 5.25 Amps flows in its primary winding's. Assume there are no internal losses.

1. Calculate the fault level which would be produced by the transformer?

2. Calculate the prospective short circuit current that would flow if a short-circuit of negligible impedance occurs across the transformer output terminals?

I am hoping you could help me as to what variables I need and formulas to use.

Cheers Wire_Wizzard
 
Wire_Wizzard said:
Dear mike your You tube page is awesome!

I however come from New Zealand and things are a wee bit different, but physics ain't To quote Admiral Montgomery Scott " you can nay change the laws of physics laddie" Star Trek. But true none the less. I have an exam preparation question and I have been hunting through all my old papers and the units on single phase and three phase transformers, but can't find a working formula or process to follow to help me answer the question and also have limited contact with my tutors.

Question: An 11kV /400v, 100kVA, three phase, delta-star transformer has a 5% impedance. When fully loaded a line current of 5.25 Amps flows in its primary winding's. Assume there are no internal losses.

1. Calculate the fault level which would be produced by the transformer?

2. Calculate the prospective short circuit current that would flow if a short-circuit of negligible impedance occurs across the transformer output terminals?

I am hoping you could help me as to what variables I need and formulas to use.

Cheers Wire_Wizzard
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I am not going to be much help at answering you question, but did notice you basically addressed your inquiry to Mike (Holt). Just a FYI, Mike doesn't actively participate in this forum, though he does start threads mostly to distribute information but seldom replies to any comments that come up in those threads. So you are typically stuck with the rest of us for replies to questions you may have, but there are many good minds that participate here, but be patient as the right person doesn't always catch a question immediatey, or may not see it at all if there is no activity in the thread to keep it a current thread.

Welcome to this forum.
 
I did not see a difference between the two questions above.

Prospective short-circuit current Ik=P/(U*1.732*5%)=100/(0.4*1.732*5%)=2.89kA
 
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