Good day all

I have a three-winding power transformer with vector grouping of YNyd11. I also have the utility short-circuit currents at the primary and secondary wingdings, and the primary-secondary, primary-tertiary, and secondary-tertiary positive and zero sequence impedances. I must calculate the three-phase and single-line-to-ground (SLG) short-circuit currents at the tertiary (delta connected winding) of the transformer.

Standard short-circuit analysis software (using ANSI C37.010) appear to ignore single-line-to-ground short-circuit current at the delta winding of a tertiary transformer. We however, need to calculate the prospective short-circuit current at this winding for protection co-ordination. Can you provide any guidance for how this can be done?

I already calculated (manually, as well as software verified) the three-phase short-circuit current at the tertiary winding by constructing the equivalent circuit for the three-winding transformer (by finding ZP, ZS and ZT) [ABB Electrical Transmission and Distribution Reference Book, fifth edition, pg 136] but have not found a way to calculate the SLG for the tertiary winding.

I am thinking this should be easy, since the purpose of a delta-connected tertiary is specifically for unbalanced faults (SLG), but I am not finding any calculation dealing with it anywhere. I am only finding the example to calculate the three-phase fault on the tertiary, which uses only the positive sequence impedances.

This was a very useful page > http://yourelectrichome.blogspot.com/2011/08/three-winding-transformers.html

Any assistance is much appreciated.