Reverse Power Relay - Function and Operation

A reverse power relay is a directional power relay that is used to monitor the power from a generator running in parallel with another generator or the utility. The function of the reverse power relay is to prevent a reverse power condition in which power flows from the bus bar into the generator. This condition can occur when there is a failure in the prime mover such as an engine or a turbine which drives the generator.

Causes of Reverse Power

The failure can be caused to a starvation of fuel in the prime mover, a problem with the speed controller or an other breakdown. When the prime mover of a generator running in a synchronized condition fails. There is a condition known as motoring, where the generator draws power from the bus bar, runs as a motor and drives the prime mover. This happens as in a synchronized condition all the generators will have the same frequency. Any drop in frequency in one generator will cause the other power sources to pump power into the generator. The flow of power in the reverse direction is known as the reverse power relay.

Another cause of reverse power can occur during synchronization. If the frequency of the machine to be synchronized is slightly lesser than the bus bar frequency and the breaker is closed, power will flow from the bus bar to the machine. Hence, during synchronization(forward), frequency of the incoming machine is kept slight higher than that of the bus bar i.e. the synchroscope is made to rotate in the "Too fast" direction. This ensures that the machine takes on load as soon as the breaker is closed.

Setting the Reverse Power Relay
The reverse power relay is usually set to 20% to 50% of the motoring power required by prime mover.  By motoring power we mean the power required by the generator to drive the prime mover at the rated rpm.  This is usually obtained from the manufacturer of the prime mover (turbine or engine).