Reverse power operation may cause damage to the prime mover. Hence, reverse power protection is a vital part of the generator protection scheme.
The reverse power relay operates by measuring the active component of the load current, I x cos φ. When the generator is supplying power, the I x cos φ is positive, in a reverse power situation it turns negative. If the negative value exceeds the set point of the relay, the relay trips the generator breaker after the preset time delay.
The typical setting for reverse power is 4% in case of turbines and 8% in case of diesel engines. The time delay can be set from 2 to 20 seconds.