Reverse Power Relay

The Reverse power relay is used to protect a synchronous generator, running in parallel, from motoring. Motoring occurs due to the failure of the prime mover such as a turbine or an engine driving a generator that is connected to the grid. The generator which is running at the synchronous speed will continue to run at the same speed. However, the power required to keep the generator running along with the prime mover will be drawn from the mains. Hence, power flows in the reverse direction i.e. bus to generator. This condition is called reverse power.

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.