Are you sure that the impedances stated are for the correct voltages? If you calculate the actual impedance values per phase using the formula Z= (%Z/100)*(KV^2)/(MVA), you get the following values -
11.5 KV, 12.5%: Z = 0.125*132.25/250 = 0.066125 Ohm
15 KV, 20%: Z = 0.20*225/250 = 0.180 Ohm
These are very disparate values, which leads me to think that the values are in fact reversed, as follows -
11.5 KV, 20%: Z= 0.20*132.5/250 = 0.106 Ohm
15 KV, 12.5%: Z= 0.125*225/250 = 0.1125 Ohm
These results appear more reasonable, as the 15 KV connection will have a somewhat higher Ohmic impedance than the 11.5 KV connection.
Recall the definition of percent impedance - it is the percentage of rated 3-phase voltage that must be applied to a winding of the transformer which will circulate rated current in the winding, with the other winding of the transformer short circuited.
As jbartos states, the impedance will influence the short circuit level - this could be a concern when the transformer is used to connect the 110 MW set, as the 11.5 KV bus duct (assumed that this is the connection method) may be braced for a lower fault level. Assuming an infinite bus on the HV side, the transformer will deliver a maximum of (250/0.2)= 1250 MVA RMS symmetrical into a 3-phase short circuit on the LV side (this assumes that my analysis above is correct for the 20% impedance applying to the 11.5 KV connection).
This and the other concerns for generator operation with this transformer need to be addressed by a series of system studies to confirm the application.
