Reverse power or motoring action of alternator is likely to happen in power plants generating or transmitting power directly to grid. In reverse power the alternator acts like an motor by taking power from the grid.
What happens in reverse power:
In normal case the alternator is driven by the prime mover (engine or turbine) and the alternator supplies power to the grid directly. While transferring power to grid, the alternator is directly attached to the grid. The prime mover maintains enough torque to maintain the alternator in generation mode.
So in generating mode the prime mover drives the alternator, alternator supply power to grid. In case of motoring mode , the grid supply power to alternator, alternator as a motor drives the prime mover.
Drawbacks of reverse power:
Revers power is not that dangerous, as there is no voltage and current involve. The only fact in reverse power is the the rotation speed of prime mover is not controllable. Because to control a prime mover the only key is to control the fuel to it. But in reverse power the alternator is driving the prime mover feeding from grid. And the grid is not controllable.
So the speed of rotation or RPM is uncontrollable in reverse power.
In turbine power plant the turbine is made for excessive rpm with lots of fluctuation, so in case of reverse power the turbine plant will not suffer. But in case of diesel or Gas engine, there is speed limitation which is very narrow. A engine rated to 750 RPM is allowed to rotated maximum at 840RPM. Beyond the permissible RPM the diesel/ gas engine will face a severe mechanical breakdown.
What to do if reverse power fault happens:
If reverse power fault happens in a power plant, the directional relay will sense it and send the disconnecting or break signal to circuit breaker to disconnect it from the grid.
But if by some means the circuit breaker is stuck, then the only option left is to to shut down the whole power plant.
More on protection system.