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.
Last edited by AdrianW; 05/23/11 04:25 PM. Reason: fingers ahead of brain!