The under frequency relays at least worked OK.

In NZ they are set to 47.8 Hertz for selected feeders and 47.5 Hertz for all feeders in substations of the local POCO Auckland distribution network, to avoid a complete blackout.

We tested all our UF relays about 2 months ago in preparation for an interesting wintertime with low hydro levels.

The product of rotation, excitation and flux produces electricty.