UR T60 differential mis-operated on an external downstream fault
UR T60 differential mis-operated on an external downstream fault
(OP)
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!
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!






RE: UR T60 differential mis-operated on an external downstream fault
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.
RE: UR T60 differential mis-operated on an external downstream fault
Thanks smallgreek!
RE: UR T60 differential mis-operated on an external downstream fault
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.
RE: UR T60 differential mis-operated on an external downstream fault
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.