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Avail Fault Current from three single-phase XFMRS

Avail Fault Current from three single-phase XFMRS

(OP)
I'm trying to calculate the available fault current for a 3-phase service. It's easy if the utility transformer is a conventional 3-phase XFMR, but I'm not sure what to do when it's three single-phase XFMRS.

I've seen it before, but in this case we have one 75kVA (2% Z) and two 37.5kVA (1.7% Z) transformers (delta configuration). I could use the largest value and calculate worst case available fault current, but is that really physically possible if the other two phases can't allow that much energy through? Will I be sizing equipment incorrectly?

Thank you.

RE: Avail Fault Current from three single-phase XFMRS

Let's assume 240 Volts. Even if the voltage is different the ratios will be the same.
75 KVA / 240 V x 0.02 = 15625 A
37.5 KVA / 240 V x 0.017 = 9191 A
The two 37.5 KVA transformers may be considered as an open delta bank forming a virtual transformer across the open side.
The available fault current of the virtual transformer will be the same as the fault current of either of the 37.5 KVA transformers. That is 9191 A.
This virtual transformer will act in parallel with the 75 KVA transformer and the fault currents will add.
The single phase fault current across the 75 KVA transformer will be 15625 Amps plus 9191 Amps = 24816 Amps.
This will be the worst case fault current.
I hope that your primary is not a wye circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Avail Fault Current from three single-phase XFMRS

(OP)
Thank you Waross!

What happens if the primary is Wye?

Thanks again.

RE: Avail Fault Current from three single-phase XFMRS

It doesn't change the secondary fault current. The issues are a different matter.
A grounded wye:delta is intolerant of any inequality or phase angle error on the primary side.
A grounded wye:delta will back-feed into a distant fault on the primary circuit.
The zero sequence impedance is three times the transformer impedance. This maens that with a bank comprised of 2% Z transformers, if the voltage on one primary phase is 6% low, a circulating current equal to full load current will flow around the delta.
Do a search on this site for more information on the idiosyncrasies of the grounded wye:delta connection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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