Grounding resistor rating ampere calculation criteria shall be considered base on equipment insulation protection (Ig >3Ic0) for high resistance NGR selection and power system stability (Refer to Machine Riddle No.5) for low impedance NGR selection. But about your problem regarding burning of NGR due to earth fault current, obviously there is not any coordination between your relaying protection system and current time withstand of grounding devices. It seems the specified NGR to previous power transformer was selected regarding power system stability consideration, so in this case, perhaps can be said in rough role: you can select a NGR with rated current two times ( 100MVA/50MVA) of previous NGR current. For time withstand selection of NGR (10sec., 1 min., 10 min.,), I said some selection criteria in above post.