Tripping 6.6 kV Motors on NPS Protection

The relays acted properly. You have to look at what causes a NPS relay to operate; i.e., they detect high levels of Negative Sequence voltages/currents, depending upon the exact type.

You didn't say what the nature of the system disturbance was, but assuming it was a single line to ground fault then the remaining two phases essentially continued to power the system as a severely unbalanced single/three phase system (depending upon the transformations between the fault and the relaying).

This in turn caused negative sequence currents to flow far in excess of the 5% trip level that the machines' protective relays may be set for. Given the fact that it reached as deep as the generator the fault must have persisted for quite a long time, long after the HT motors tripped.

Did something take out a tower or get tangled in the conductors?