To check the stability of a Numeric/Digital differential relay for Generator, 0.8 A was applied from both sides (line side & neutral side) in all 3-phases at opposite directions. Differential current was zero as appeared on relay while restraint current at relay was 1.61A
Can anyone please explain this text to me? It's understandable why differential current is zero but how is restraint current 1.6 A?

 

Responding to Vassy's 08-Jan-09 (08:36)...

If test was conducted by single-phase secondary-injection and values obtained were the same on all three phases, problem is caused by:

1) If a new installation, then wye-midpoint of one set of CTs is opposite that of the other set! That is, currents through restraint-coils of relay are additive instead of subtractive, thus bypassing the operating differential or operating-coil!

2) If existing installation, then injection-current has been reversed on one set of CTs!

Additional info available upon request!

Regards, Phil Corso ([email protected])

 

Vassy, further to my 09-Jan-09 (09:12) reply, see Control List Thread # 1026247191.

Regards, Phil Corso

 

Check one by one individual outgoing feeders diff. CT current at terminal block. if you found difference then check the resistance and impedence of particular o/g feeder's CTs.may be one of the CT have low value.