Power network short circuit current calc

AI Thread Summary
The discussion focuses on calculating short circuit current in a power network involving parallel generators and transformers. The user modeled a fault scenario in software and sought to verify results through manual calculations, noting discrepancies in textbook approaches that often ignore source and line impedance on the 33kV side. They questioned the rationale behind this simplification and referenced IEC60909, which includes source impedance in its calculations. The user found the IEC60909 methodology confusing, particularly regarding how 33kV impedance is perceived from the 690V side. Ultimately, they resolved their inquiry with assistance from the forum.
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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 modeled 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 textbooks 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 textbooks 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?
 
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