Protection on transformer winding grounding fault

The single phase grounding fault of transformer's star winding is analyzed with the ampere turns balance.The differential currents for the longitudinal differential protection and the restricted earth protection are derived in term of ampere turns.It shows that the restricted earth protection is very sensitive when the fault is near the neutral point of the star winding.The longitudinal differential protection can also protect such faults though it is less sensitive than restricted earth protection.There is no this kind of fault records in the fault statistics of China.The necessity of adding the restricted earth protection should be discussed.