Tertiary Delta Winding Earthing for IBT 500 kV

good afternoon.. i wanna asking to you all..

how to ground the coil in the transformer delta tertiary, and as we know that the coil is not delta neutral point ?

and what is the comparison between the coils which grounded with not grounded?

and what formula is used for grounding the coil ?

I just want to know..

I am sorry to ask for many questions, and also sorry my bad english..

thank you..