Folks,

I understand why and where RPR are used. What I do not understand is how they determine the direction of power flow. Can anybody help me?

Thanks

Voltage drop across a resistor.

Simple.

Tesla, that describes a current shunt.

Where I am familiar with reverse power relays is when you have multiple paralelled generators or a generator tied to the grid. If the prime mover slows down, the slower generator turns into a motor and consumes power instead of producing power. This motoring action can then damage the prime mover.

Looking at the tech sheets, there are connections to current transformers and potential transformers, but how do you determine the direction of power flow?

Larry

I am guessing here but I bet it is by seeing which one has the higher voltage, by looking at that resistor.

Power will always flow from a higher voltage to a lower voltage and if you compare volts in and out of a resistor you can see which end is higher. We are talking about a pretty small delta.

LarryC

The grid is an infinite bus.

All big prime movers drive synchronous alternators -- and they stay in lockstep via their excitation.

Frequency control and synchronization were solved by Tesla.

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.