Step down, Step Up…….
It is a transformer, it has a primary and a secundary, two levels of tension and same power (if excluded the losses in the iron core and in the wiring), so if you have the same power and diferent tension levels, off course the wires are thicker in the side of the lower tension.
It is a reversible machine, you must respect the tension and current rating, it doesn´t mather if you use it as a step up or a step down transformer.
You get a transformer off let’s say 15kV/0,4kV 630kVA and conect a load of 630 kVA (many loads used in 0,4kV) using the transformer as step down, than you get in the secudary (0,4kV) 909 A .
Later on you decide to use it as a step up transformer, then you conect 0,4kV to the former secundary, now is the primary (primary, secundary it is not the level but were you conect the power input and power output), you also conect a load 630kVA in the secundary (15kV) side, off course it can not be the same load used in the step down operation, same power but diferent level of tension. Then you get in the secundary (15kV) 24,2A.
Same power transfer from primary to secundary (630kVA) working as step up or step down, same efficiency, same insulation problems, same losses.