KD relay misoperation

[cda2000]

We had a KD4 relay misoperation that is the relay tripped with no fault on the line. I tested the relay - scanned the characteristics for ABC, AB, BC, CA. The only problem I could see was at CA test -the relay was constantly overreached by 8.7%. Also for that test the 3 phase contact closed too which is not right. However in order to see the overreach the test current was at least 4A. Based on SCADA reports the secondary current before the relay operation was 0.5A. At this level of the load I am not able to replicate the problem. Has anyone had any similar experience with this type of relay.I am also attaching the test file. Thanks.

 

[dw51]

A co-worker of mine encountered a KD problem a few years ago - misoperation in service but no problem when bench tested. Turned out to be an internal fault to the relay frame, high enough in impedance not to effect supply voltage but enough to skew operation. Westinghouse instruction manual doesn't call for testing in case, but once he put the KD into a case and grounded the case to his relay test set, the misoperation became clear. This may have nothing to do with your problem but just a shared experience.