1-How did you read the mentioned Zs? 2-The vector group of mentioned transformer is Dyn11 probably, is it correct? 3-If yes, the supplementary earth conductor of MV cable is not a part of primary earthing system; because the primary delta connection of same transformers are ungrounded ordinary and it has used as cable shielding probably. 4-The secondary earthing system selection (TNS, TN-CS,) could not affect the primary impedance. 5-The electrical loss connection due to electrodynamics short circuit forces and some earth path born occurring due to sever short circuit is possible and these phenomenon can be caused that Zs increasing.