Earthing system type that is considered in main substation is impedance grounding. Also the related resistor is not connected to star point of that mentioned transformers. They installed on star point of two zigzag earthing transformers that are connected to main bus bars of substation.
Indeed the earthing point of electrical system; because of some location problems are moved from typical place to substation room.
In the other hand, all main substation incomings are equipped by many earth fault relays (51N) that shall be protected the main bus bar against probable earth faults on it.
Next investigations shown that earth fault relays are not able to do works that assigned for it.
How can you explain the reason of that protection system inability?