In Dyn connection with solidly neutral grounding system, the fault current is controlled mainly by the leakage reactance of the winding, which varies in a complex manner with the position of the fault. 
The variable fault point voltage is also an important factor, as in the case of impedance earthing. For faults close to the neutral end of the winding, the reactance is very low, and results in the highest fault currents. The variation of current with fault position is shown in below Figure. 
As you see for faults close to the neutral end of the winding primary the current protection systems cannot have suitable function against secondary fault which can be very sever. Therefore in some case primary side protection cannot cover all secondary fault situations.