Field failure protection has nothing to do with neutral currents at all.
If an alternator operating in synchronism with other machines in a power system looses its excitation The machine will continue to operate delivering the same electric power, with a slightly higher than synchronous speed (corresponding to negative slip) as an induction generator. The solid forged rotor and the limited damper winding of the machine will result in excessive heating of the rotor, which is now carrying a slip frequency current of large magnitude. The machine will also draw large quantum of reactive power from the system to cause excessive stressing of ststor winding also.
The condition is detected by a LOF protection relay by measuring the impedance seen from the machine terminals to initiate tripping.
