Transformer Impedance Calculation

Hi all, I have a very fundamental question to ask. To compute a 3 phase fault current of a circuit supplied by a 1MVA transformer 22kV/ 400V with 5% impedance without using PU method, I understand it goes:

TF impedance = 5% x 400^2/ 1x10^6 = 0.0084 ohm

The 3 phase fault current per phase = 400/ 1.732/ 0.0084= 27.5kA

I am puzzled by the TF impedance calculation.. why is there no 1.732 factor as with all 3 phase power calculations in this instance? Sorry for the very layman question asked here. But am really confused. Thanks.


asked 5/26/2018

1 Answer

kVA / Voltage x 1.73 / TX Z%
1000 / 400 x 1.73 / 5% = 28,9 kA
answered 4 months ago Paddy Keys 3
Paddy Keys

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