UR T60 differential mis-operated on an external downstream fault

[pwrtran]

UR T60 differential mis-operated on an external downstream fault. I looked into the relay settings and found several things I thought may cause the T60 mis-operated from most likely to least likely in the following order:

1) SRC 1 doesn't get any frequency and phasor measurement
2) in transformer winding settings, "Not within Zone" for both winding 1 & 2 of a Wye/Ground-Wye transformer, which I think should be set to "Not Within Zone" for Winding 1 and "Within Zone" for Winding 2
3) Reference CT was assigned wrong winding, but I don't think it will affect the differential for a fault at 4 kilometer away downstream. It may affect the relay calculation if any CT saturates.
4) transformer rating was input as ONAF rating instead of ONAN rating

Are the above findings hole the statement true? Any other settings can also cause T60 mis-operation?

Thanks in advance!

 

[DTR2011]

I am not sure of the specifics, but in general you want to ensure that the relay is dealing with the zero sequence elimination.

I am assuming that the CT's are wye connected.

The best check for this is during relay testing. Place an out of zone ground fault and ensure that the relay restrains for this.

Likely you have an event report for this. You can play with the settings and use the event report COMTRADE file to test for this scenario.

 

[pwrtran]

Agreed, that was what I found in 2), the zero sequence was not compensated in the relay calculation due to the wrong "Not Within Zone" settings for Wye-ground winding 2. The fault was a 3-phase fault, and it maybe very likely started involved the ground as well.

Thanks smallgreek!

 

[rcw retired EE]

#2 probably caased your trip. A grounded wye winding has grounding within the zone.

The rest of the items on a T60 will affect some metering style readouts but will not affect the protection except choosing a different reference winding can affect the sensitivity.

The false trip may also be casued by incorrect CT connections or compensation settings. Check your record files and verify the relay was seeing the correct currents during the fault.

 

[RRaghunath]

I am not familiar with T-60.
In some other make relays (7UT6XX of Siemens etc.) 'not within the zone' would be allowed provided trafo neutral CT current signal is also wired to the relay so that the zero sequence current in the phase CTs can be compensated by the neutral current by the relay algorithm.
This method is in fact recommended and makes the relay sensitive for in-zone earth faults.