Zig Zag Transformer Sizing

[Blackberry77]

Hi,

I'd like a sanity check on my calculations for a zig zag transformer. I followed IEEE Std C62.92.4

System Voltage - 44 kV
Using 100 MVA base

Base Ohms = (44000)^2/100000000 = 19.36 Ohms
Base Amps = 100000000/(44000)*(1.732) = 1313 Amps
Three Phase Fault Duty = 6.4 kA
Three Phase Fault MVA = 44000*1.732*6400 = 415 MVA
Z1=Z2=100/415 = 0.241 PU
Desired Ground Fault Current = 770 A
Ground Fault Current Ig = 770/1313 = 0.586 PU
Io = 0.586/3 = 0.195 PU
Z(total) = 1.0/0.195 = 5.128 PU
Z(total) - 2Z(1) = 5.128 - 0.241 - 0.241 = 4.646 PU
Zo = 19.36*4.646 = 89.95 Ohms Per Phase
Short Time Rating = (44000/3)*(770) = 11.3 MVA

Thanks in advance!

 

[wcaseyharman]

I tracked until your very last equation - I would have thought it’d be 770*44000/Sqrt(3) =19MVA to account that it’s providing single phase fault current.