Phase distance relay overreach
Phase distance relay overreach
(OP)
Am I thinking about this correctly? If you have an arrangement with a delta-wye-grounded transformer feeding a plant and a line to ground fault happens on the low side, the phase distance relaying on the high side can overreach to the low side due to load current on the other two phases causing the distance relaying to overreach.
As - phase A secondary - As fault causes fault currents on Aprimary and Bprimary
Bs - phase B secondary - Bs load causes currents on Bprimary and Cprimary
Cs - phase C secondary - Cs load causes currents on Cprimary and Aprimary
Total -
Aprimary + Bprimary (fault current)
Bprimary + Cprimary (Bs load current)
Aprimary + Cprimary (Cs load current)
End result - Aprimary + Bprimary is higher with the load and might cause the phase distance relaying to overreach.
As - phase A secondary - As fault causes fault currents on Aprimary and Bprimary
Bs - phase B secondary - Bs load causes currents on Bprimary and Cprimary
Cs - phase C secondary - Cs load causes currents on Cprimary and Aprimary
Total -
Aprimary + Bprimary (fault current)
Bprimary + Cprimary (Bs load current)
Aprimary + Cprimary (Cs load current)
End result - Aprimary + Bprimary is higher with the load and might cause the phase distance relaying to overreach.






RE: Phase distance relay overreach
But- to be sure more information is needed regarding your system.
RE: Phase distance relay overreach
RE: Phase distance relay overreach
RE: Phase distance relay overreach
An impedance relay looking into a xfmr is usually not set to about half the impedance of the xfmr, so it will not see through the xfmr or trip for secondary faults or inrush currents. e.g. the impedance relay is set to 5% of the base impedance of the xfmr, so you need 20 per unit current for it to trip. The load current is negligible in this perspective.
J. Horak, P-R Engineering, Colorado
RE: Phase distance relay overreach
Some zones are set to not see through the transformer. Other zones, with longer delays, may be intentionally set to reach through to provide last ditch back up protection.
RE: Phase distance relay overreach
RE: Phase distance relay overreach
RE: Phase distance relay overreach
J. Horak, P-R Engineering, Colorado