First-cut check for a two-winding autotransformer (series + common, no galvanic isolation). Tender / estimation grade — verify with the manufacturer.
Method (step-down autotransformer): series voltage = HV−LV, common = LV; series:common turns = (HV−LV):LV. Common-winding current Ic = ILV − IHV. Self (transformer) rating = S·(HV−LV)/HV — the autotransformer only processes this fraction of throughput, which is why it is smaller. Short-circuit currents use the assumed %Z; the autotransformer's actual %Z is lower than an equivalent two-winding, and the exact IEC 60076-5 value needs the coupled-winding equivalent circuit — treat the impedance figures as first-cut.