Why neutral of the transformer is grounded?

Neutral of transformer is grounded to avoid fault current it carries fault current .when any fault is occures in tranfoermer current suddenly starts rapidly increasing which may demage the transformer.Neutral of transformer will carry this  faulty current and to ground.

Transformer neutral graounding was depends upon the purpose of the transformer in the network.

Normally transformer neutral was grounded mainly to protect the transformer from earth faults. This was done by installing a protection CT in Neutral. This protection is achieved either by REF or SBEF.