In case of generator feeds power via a delta star transformer, an Instantaneous Relay is used for the same purpose. In the former case, the earth faults relay is required to be graded with other fault relays in the system. This is the reason why Inverse Time Relay is used in this case. But in the letter case, the earth fault loop is restricted to the stator winding and primary winding of the transformer, hence, there is no need of grading or discrimination with other earth fault relays in the system. That is why Instantaneous Relay is preferable in the case.