SIZING OF GROUNDING TRANSFORMERS

[genhead]

I have 2 x 6.6kv, 1MW diesel gensets stepping up through a 2.5MVA 11/6.6kv YnD transformer. The application is standby/peak lopping.
I intend to install an earthing transformer at the 6.6kv terminals of the step-up transformer , and leave the generator neutrals disconnected.
How do I size the earthing transformer ?

 

[230842]

Generator neutrals are gennerally connected to ground trough a limiting fault current resistor which allows fault detection. Earthing transformer sould take the same fuction and it must whithstand generator side voltage Vg and fault limited current If, so an apparent power:
sqrt3 x Vg x If. Julian