Briefly an
earthing transformer provides a ground path with relatively
low impedance, so that the system neutral will be near ground potential. Also limits
the transient over voltage spikes and provides the ground fault current during a line-ground fault.
These transformers are relatively small, no true kVA rating is normally specified unless you are expecting it to use it to provide auxilliary power. Sometimes what's known as a
zig-zag grounding transformers are used - these are smaller in size for the same zero sequence impedance.
As I said before it's a bit hard to explain these in detail...