I have a power network, two parallel generators which feed 2 parallel delta-wye transformers with a bus tie inbetween, from 690V to 33kV. These two transformers feed a bus which a whole bunch of 33kV/690V DELTA-WYE transformers hang off for distribution. There was a fault the other day at the bus which is fed from one of these 33kV/690V step down transformers which I modelled in some power network software to determine the peak value of asymmetrical fault current. I decided to try and compute it by hand (something I have not done in a very long time) just to confirm the result of the simulation. I looked at some text books and I noticed all of them ignore source and line impedance (33kV side) before the 33kV/690V distribution transformer and state the current is limitted purely by the 33kV/690V impedance + the bus bar (which it feeds) impedance. I am no specialist in power protection but I have my own theories why the source impedance can be ignored. Can anyone shed some light on why the text books do it? I also found IEC60909 which does not neglect source impedance when performing a short circuit calc. It is quite confusing as to the way they do it. They seem to reference the impedance on the 33kV side of the transformer to 690V. Can anyone explain how the impedance on the 33kV side is seen from the 690V side?