ABB REL670 2.2 IEC, Applications Manual

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Due to the resonance/high resistive earthing, single phase-to-earth faults give
relatively low fault currents. However, it also raises the voltages in the healthy
phases drastically, which increases the risk for a second earth fault. When the
second fault occurs on another feeder, the two faults are commonly called a cross-
country fault.
Regardless of the practice to trip, or not trip single phase-to-earth faults a cross-
country fault may still occur before the earth fault protection has had the time to
trip.
Cross-country faults can generate relatively high currents and need to be tripped
automatically and fast. The strategy is still often to trip only one of the two faulty
feeders during the cross-country fault and then continue to run the system. The
PPL2PHIZ function is designed to enable this strategy, being able to properly
select the phase which shall be tripped first based on set phase preference scheme.
If the cross-country faults are very far apart or the network is relatively meshed, it
can be difficult to obtain phase preference. The PPL2PHIZ function will still
enable tripping, although it might be unselectively.
If fault current levels are low, it might also be a good idea to consider earth fault
protection as a backup.
Figure 245 shows a typical cross-country fault situation. Figure 246 shows phase
voltage relations before and during a single phase-to-earth fault.
L1N
L3N
Load
Load
en06000550.vsd
IEC06000550 V1 EN-US
Figure 245: A typical cross-country fault situation
Section 8
1MRK 506 369-UEN B
Impedance protection
450 Line distance protection REL670 2.2 IEC
Application manual