We have got three numbers of single phase 200MVA, 420 / 21 kV, 50 hz, YNd11 connected via isolated phase bus duct to 500MW Generator. The 21kV single phase transformers are connected via isolated phase bus duct (IPBD) to form the delta configuration for the generator transformer. Pl. provide guidance on how to calculate the short circuit level for the delta run of IPBD. Is there any IEEE or IEC standard for the same?