A reverse power relay is a wattmetric type of device that closes a contact and trips the generator breaker when the kW (MW) from a generator to the busbar goes negative, in otherwords goes into "motoring".
Motoring is not allowed for any generator as the prime mover will be affected: In a steam turbine there will be insufficienr steam flow to maintain temperature balance, if a diesel it will act as an air compressor, a hydro may begin to cavitate...
Usually a time lag of not more than 10 seconds is applied.
Motoring is caused by the prime mover itself, or the governing system having shut off the fuel (water) for some reason, or an operator error, usually when taking the unit off line, or even just after synchronizing to the busbar.
for steam turbines, Reverse Power relay logic has been added (recomended) to provide verification that the steam flow has been stopped BEFORE allowing the generator breaker to be open on a turbine trip.
