Generator current differential trip

[electmags]

I'am running a hydro generating plant for the pass 12 years, adout a week ago I have one of my generating unit triping on 87G Generator current differential relay. For this pass week I have conduct all type of test on my stator,CT's, relay and wiring with no luck.When the fault started I ran the unit for 3 hour before I got the fault again, but now I'am trip as soon as the unit breaker close to line. We could start the unit excit and close the breaker off line whit no trip. Please I need help!

 

[RRaghunath]

It seems to be a classic case of one of the 87G CTs being open circuit. When one of the CT secondary wires is open / loose, what happens is spill appears at the relay coil and increases to the trip value as the load on the generator is increased. That is the reason why at low loads the trip doesn't take place.

electmags! appreciate if you can check and give feedback to the forum.

 

[swn1]

*Any* fault in the secondary wiring will trip the 87 when current flows but not before. A real stator fault would trip before the breaker was closed.

Open CT circuit is one possibility, but grounded "dot" CT leads are another possiblity. Even a ground loop in the common leg is a (more remote) possible cause.

You could also have developed a problem in the restraint winding of the relay itself.

If you have single-phase differential relays, try swapping them and see if the problem follows the relay or stays with the phase.

See if somebody left a shorting screw in one of the CT shorting terminal blocks.

 

[electmags]

Thanx for inputs, We have the unit running it seens I had problem with cross current xfmer for excitation.