Short circuit value for 3 TX in //
Short circuit value for 3 TX in //
(OP)
Hello,
I assume that "the short circuit power" at the MV side of a 11kV/415V transformer is 250 MVA.
If the TX is rated at 1500 kVA, then the short circuit at the LV terminals is "calculated" to be 34 kA, approximatively.
Put 3 TX in parallel, and the short circuit should become: 3x34= 102 kA.
The question is: someone is saying that even though the above calculations are correct, they do not correspond to reality. In reality, the sc would be "around 65 kA" only.
Which one is correct? The panelboard can cost double in one alternatvie than the other.
I assume that "the short circuit power" at the MV side of a 11kV/415V transformer is 250 MVA.
If the TX is rated at 1500 kVA, then the short circuit at the LV terminals is "calculated" to be 34 kA, approximatively.
Put 3 TX in parallel, and the short circuit should become: 3x34= 102 kA.
The question is: someone is saying that even though the above calculations are correct, they do not correspond to reality. In reality, the sc would be "around 65 kA" only.
Which one is correct? The panelboard can cost double in one alternatvie than the other.






RE: Short circuit value for 3 TX in //
RE: Short circuit value for 3 TX in //
Full load amps on a 1500 kVA transformer is 2,087 A. Since calculated fault current is 34,000 A, this gives you a total impedance (source plus transformer) of 6.14% (2/34).
How much impedance for transformer. I'll assume 5%. This leaves 1.14% for the source.
If I parallel 3 of these babies, you'll get transformer impedance of 1.67%. Total impedance with 3 transformers is 1.14 plus 1.67 or 3.3%.
Resulting fault current = FLA/Z
or = 2087/0.033 = 63,238 A
You can go back with actual transformer impedances if they are different than what I assumed.
RE: Short circuit value for 3 TX in //
RE: Short circuit value for 3 TX in //
The source impedance is also expressed on the same base.
When you parallel the 3 transformers, treat them like 3 impedances in parallel. The result works out correctly as long as you follow the base quantities you selected.