ABB RELION 670 SERIES Applications Manual page 488

Section 8
Impedance protection
482
(Petersen coil) or high resistive grounded systems where phase preference based tripping
is required for so-called cross-country faults, two simultaneous single phase-to-ground-
faults in different phases and on different line sections.
Due to the resonance/high resistive grounding, single phase-to-ground faults give
relatively low fault currents. However, it also raises the voltages in the healthy phases
drastically, which increases the risk for a second ground 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-ground faults a cross-country
fault may still occur before the ground 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 ground fault
protection as a backup.
Figure 243 shows a typical cross-country fault situation. Figure
relations before and during a single phase-to-ground fault.
ANSI06000550 V2 EN
Figure 247: A typical cross-country fault situation
CG
AG
1MRK 506 369-UUS -
244 shows phase voltage
Load
Y
Load
Y
ANSI06000550_2_en.vsd
Line distance protection REL670 2.2 ANSI
Application manual