This sounds like a generator step-up transformer from the rating and large ratio. From the transformer data you've given us, and with the assumption that the impedance is in the region of 15% or so, you should be able to directly connect a mobile generator to the LV winding, short the HV winding, and circulate current through the transformer as Ralph suggests.
You won't be able to circulate rated current in the windings without using either an HV generator or interposing a distribution transformer between the generator and the transformer-under-test, but an LV generator should circulate enough current to allow you to check your CTs and relay without the additional complexity. Circulating full rated current in the windings will require a very powerful test source.
Guessing a couple of values and using your supplied data:
I've assumed
Transformer Z=15%
Mobile generator supply 415V
Rated LV current = 30E6/(sqrt3*12.47E3) = 1389A
LV volts for rated current into HV S/Cct = 12.47E3/0.15 = 1.87kV
LV Current with 415V on LV winding and HV S/Cct = 1389* (415/1.87E3) = 309A.
309A is just within the capability of a 250kVA set, but the genset will probably object to a step load change of 90% of rating. I would probably go up to a 300 or 350kVA unit. The rental cost is not significantly different.
Interposing a typical 11/3.3kV distribution transformer, with the 415V source to the 3.3kV winding and the transformer-under-test's LV winding to the 11kV winding, the current in the LV winding of the transformer-under-test rises to about 1030A, which is about 75% of rated current, at the expense of increasing the load on the test generator to 3500A @ 415V. Sets of this size certainly exist, but arranging such a test is a fairly major undertaking. The directly connected LV generator will show up any CT polarity problems in a sensitive relay such as transformer differential anyway, so you would be making a lot of work for little gain.
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