Short circuit value for 3 TX in // 2

[AusLee]

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.

 

[davidbeach]

The fault current would only triple if your MV side source were an infinite bus. With one transformer (assuming all three are equal), your impedance to the fault is Z_s+Z_t; with three, your impedance to the fault is Z_s+0.33*Z_t. I leave it as an exercise for you to calculate Z_s and Z_t.

 

[magoo2]

Let's do a back of the envelope calc.

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.

 

[AusLee]

One question please, with the 3 TX in parallel, why you consider only one In = 2087, not 3 x In ?

 

[magoo2]

When you do a calculation like this, you select a kV and MVA base. It's a per unit system. It's convenient to go with the values for a single transformer. With these 2 quantities, the base current is the value for one transformer, or 2087 A.

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.