ECN Forum Forums General Topics of the Trade General Discussion Area Reverse Power Relays how do they work?

Reverse Power Relays are indeed used to protect an alternator from 'motoring' in case of failure of the prime mover.

This can't be done by anything as simple as using a resistor inline - we are dealing with AC here, the direction of the current changes 60 times a second anyway!

These devices do have connections to both CT's & VT's. They use the voltage reference as a 'polarising voltage' ie. the current in L1 phase is polarised by the voltage L2-L3 etc. By analysing the angles between the polarising voltage & current it is possible to work out the direction of the real power (W) flow. This is the basic principle of operation of a reverse power relay & also of a directional overcurrent relay.

Remember also, when paralled against an infinite bus:
Prime-mover Torque = Real Power Output
Excitation = terminal voltage = Reactive Power Output

Either of these may be imported & exported independently on the same machine.