
Transformer Math Questions
Received a Pair of similar questions involving current calculations on Transformers, but I'm having trouble figuring out their math.
A primary injection is to be performed on a transformer rated 138kV/25kV, 2500kVA, 8%Z using a 480V generator connected to the high side with the low side grounded. What is the expected current on the high side?
a) 18.116A
b) 12.5A
c) 226.45A
d) 0.787A
e) 1.36A
And:
A primary injection is to be performed on a DY transformer rated 138kV/25kV, 2500kva, 8%Z using a 480V generator connected to the high side with the low side grounded. What is the expected current on the high side?
a) 0.262A
b) 0.455A
c) 0.787A
d) 1.36A

Originally Posted by
david.hatch
Received a Pair of similar questions involving current calculations on Transformers, but I'm having trouble figuring out their math.
A primary injection is to be performed on a transformer rated 138kV/25kV, 2500kVA, 8%Z using a 480V generator connected to the high side with the low side grounded. What is the expected current on the high side?
a) 18.116A
b) 12.5A
c) 226.45A
d) 0.787A
e) 1.36A
And:
A primary injection is to be performed on a DY transformer rated 138kV/25kV, 2500kva, 8%Z using a 480V generator connected to the high side with the low side grounded. What is the expected current on the high side?
a) 0.262A
b) 0.455A
c) 0.787A
d) 1.36A
For the first one:
1. Divide kVA by primary voltage to find FLA
2,500,000/138,000 = 18.11
2. Divide FLA by impedance to find short circuit current
18.11/(.08) = 226.44 A
3. Calculate ratio for 480V and multiply by short circuit current
226.44 A * (480/138000) = .787 A
For the second one, just use three phase voltage in the FLA calculation:
2500kVA/(sqrt(3)*138kV) = 10.45924 A
10.45924/(.08) = 130.7405 A
130.7405 A * (480/138000) = .4548 A

Originally Posted by
SecondGen
For the first one:
1. Divide kVA by primary voltage to find FLA
2,500,000/138,000 = 18.11
2. Divide FLA by impedance to find short circuit current
18.11/(.08) = 226.44 A
3. Calculate ratio for 480V and multiply by short circuit current
226.44 A * (480/138000) = .787 A
For the second one, just use three phase voltage in the FLA calculation:
2500kVA/(sqrt(3)*138kV) = 10.45924 A
10.45924/(.08) = 130.7405 A
130.7405 A * (480/138000) = .4548 A
Ok, that just confused me. Is the reasoning for the different calculations based on an assumption that the first question is single phase xfmr, while the second one specifically calls out DY?
I thought FLA calculation was independent of single phase or three phase with FLA=(KVA*1000)/(Sqrt(3)*V(LL)). Which as I type this would answer my confusion as long as the assumption is the first XFMR is single phase.

Originally Posted by
Kalbi_Rob
Ok, that just confused me. Is the reasoning for the different calculations based on an assumption that the first question is single phase xfmr, while the second one specifically calls out DY?
I thought FLA calculation was independent of single phase or three phase with FLA=(KVA*1000)/(Sqrt(3)*V(LL)). Which as I type this would answer my confusion as long as the assumption is the first XFMR is single phase.
Sorry, I was assuming first transformer is single phase. Second is assumed to be 3phase because of the DY. That is the only difference I see in the two questions.

Originally Posted by
SecondGen
Sorry, I was assuming first transformer is single phase. Second is assumed to be 3phase because of the DY. That is the only difference I see in the two questions.
I agree with the assumption of the single phase transformer, no 1.732 in calculation. If that was not the case then the two questions would have the same answer.
That is very tricky that they would ask those two questions on the same test worded almost identically?
You also have to pay attention to the fact that they are asking for the primary Isc and not the secondary.
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