21.What is reverse power? How it is going to affect the prime mover if it is not tripped during reverse power condition

Reverse power situation arises due to some fault in the prime mover. When the generator prime mover is unable to run in its synchronous RPM due to some mechanical failure or Clogged F.O Filters, governor will not be able to restore the RPM and speed of the engine will tend to drop. But the generator is synchronise generator and as long as it is connected to a multi grid or common busbar where more than one generators are connected, it will not allow to rotate RPM below than its Synchronise Speed. When the prime mover fails to rotate at enough speed, generator will start withdrawing power from the multi grid/common bus and continue to rotate at the same RPM as a synchronise motor. At this condition the generator is no more delivering power to the multi grid/common bus, instead it is withdrawing power from the grid/bus to maintain its RPM. This is called reverse power.
Engine driven generators have mechanical protection systems that shut down the drive in the event of mechanical failures such as bearing overheating, luboil low pressure or cooling water high temperature. During the reverse power situation, the generator turned motor will drag the troubled/crippled prime mover to rotate at its synchronous speed which will result with further damage to the engine. Motoring these units also poses a danger of explosion and fire because of unburnt fuel.At this condition the generator turned motor will draw huge current from the Bus bar and cause over current tripping of the healthy generators which will eventually end up with black out.