Parallel Generator Reverse Power Protection

Thread Starter


Joined Jun 20, 2009
I'm designing a generator system and will be running two generators in parallel with each other. I need to have some sort of reverse power protection so that if one generator/engine fails while they are running together it trips a breaker causing one or both generators to disconnect from loads.(When two generators are running parallel if one engine fails it's generator will act as a motor, spinning the engine with power generated by the other generator)What I have seen is that a standard reverse power relay costs about $750 and I will need one for each generator. I'm trying to come up with an inexpensive way to do this. In my system it is OK if all power to loads turns off in event of fault so it seems there should be a cheaper way to do this. Please let me know if any of the the attached diagrams will work. Please only respond if you are sure.

The generators are 3 phase synchroneus, brushless,120/208volt 25 KW nameplate amp rating:87A
Relay #1
when both generators are parallel,there is no voltage between phase A and A so no current flows between generators. However, when one generator/engine fails the generated voltages become 180 degrees out of phase and 240 volts are between phases this causes the the current relay to energize closing the normally open contacts and therefore tripping the shunt trip breakers of both generators
- Should I have one current relay per phase?
- If not, is it necessary to have jumpers across all three phases or just the one with the relay?
- what should be the trip rating of the current relay? Does it need to be adjustable? If so, what range of setting should it have?
- Does it need a time delay?

When one generator fails it seems that current will reverse thus phase will also reverse causing normally open contact to close, tripping the shunt trip breaker of faulty generator.
( these reverse phase relays say they sense phase loss or phase reversal and discount regenerated voltage{ voltage that goes through load and comes back on another phase})

Thank You,
JoshuaView attachment 10270