Tap position VS Impedance

The transformer impedance will indeed affect the fault levels, but it won't vary very much,  3 % to 5% is the right sort of range unless the transformer is very large or very small, and for a given maker and kVA that figure will change by  zero point something of a % with tap shifting you will need, which will be 10% at most, probably less, and with any significant distance, the impedance quickly becomes dominated by the cables on the LV side.

Do not overlook the value of a 'death or glory' fuse as the first line of protection, limiting the energy that reaches your expensive switch gear and breakers during  dead short, and maybe allowing them to be a bit smaller.

Also you may find the fault current is slightly reduced  by the impedance on the 11kV side - clearly to assume an infinite PSSC  (usually expressed as so many MVA of fault level, we do seem to love mixed units) is the safest assumption,  but not always a realistic one, and may be the difference between one size and the next one down.

The transformer impedance will indeed affect the fault levels, but it won't vary very much,  3 % to 5% is the right sort of range unless the transformer is very large or very small, and for a given maker and kVA that figure will change by  zero point something of a % with tap shifting you will need, which will be 10% at most, probably less, and with any significant distance, the impedance quickly becomes dominated by the cables on the LV side.

Do not overlook the value of a 'death or glory' fuse as the first line of protection, limiting the energy that reaches your expensive switch gear and breakers during  dead short, and maybe allowing them to be a bit smaller.

Also you may find the fault current is slightly reduced  by the impedance on the 11kV side - clearly to assume an infinite PSSC  (usually expressed as so many MVA of fault level, we do seem to love mixed units) is the safest assumption,  but not always a realistic one, and may be the difference between one size and the next one down.