ABB RELION RET670, Applications Manual

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8.20.2 Application
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The Phase preference logic function PPL2PHIZ provides a supplementary phase
selection to the High speed distance protection ZMFPDIS (21). The application is for
resonance (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 249 shows a typical cross-country fault situation. Figure 250 shows phase
voltage relations before and during a single phase-to-ground fault.
Section 8
1MRK 504 163-UUS A
Impedance protection
496 Transformer protection RET670 2.2 ANSI
Application manual