Reverse Power

Assume that generator-1 is running in parallell with generator-2. If the turbine of generator-1 trips, then generator-1 must be disconnected from the busbar immediately.If it is not disconnected, it will draw power from busbar and continue to run as a motor. This situation is called Reverse power which can severely damage the turbine. Reverse power relay monitors the direction of power flow and issue trip signal if the power flow is reversed.

Try this:

Normally while tripping a generator (maybe because of fault or for a shut down) a certain sequence of tripping is followed. It means that the Gen circuit breaker is not tripped first, rather the Turbine would be tripped first, then the field exitation and then only the Gen CB. This is because, if the Gen CB would have been tripped first, then the electrical load on the Gen would decrease and hence the turbine would accelerate at a higher speed and this could lead to the collapse of the turbine. Hence the turbine is tripped first. Now when this is done, the rotor would be rotating due to its own inertia. This situation would lead to the Gen behaving like a Motor, by drawing power from the supply mains. This means that the power starts flowing into the machine rather that flowing out. The machine starts drawing huge current from the supply grid. The stator & rotor experiences currents which are higher than what they are supposed to carry and this could damage the stator and rotor.This is the situation of reverse power flow. Its also called as low forward power flow.

Reverse power flow/ Low forward power relays are used to protect the machine under such circumastances. Normally the reverse power relays are set at 5 to 10% of the rated gen current and the Low forward power relay is set at 2% of the rated gen current.

Hope the above clarifies ur query.

Rgds,