ABB REL670 2.2 IEC Applications Manual page 458

Section 8
Impedance protection
452
However, it should be considered that a quite substantial residual current can also
appear temporarily at the onset of a single phase fault. This is why detection of
cross-country faults, using residual current, is typically time delayed. The time
delay should then be long enough to cover the duration of the transient residual
current.
Even though there is no considerable current in the phase where the external fault
is, the voltage magnitude in this phase will decrease more or less. This voltage drop
will be the only lasting sign of a fault in this phase, as seen from the relay. The
phase with the fault on the protected feeder will of course also experience a voltage
drop. The healthy phase voltage will stay around its rated value.
Using the voltage drops, it is possible to detect the two faulty phases of the cross-
country fault.
Another advantage with using voltage, and voltage only, is that it will be the same
for all the feeders on a bus and similar for other buses. This will give the relays a
better opportunity to detect the same two faulty phases. This is important since
phase preference can only be obtained when all relays detect the two correct
phases.
The PPL2PHIZ function is using the voltage criteria described above. Once the
faulty phases are detected, together with residual current, preference may be
obtained and transferred to the distance protection by activating one of three
enabling signals, one for each phase. Technically, the three signals are first coded
into a word together with three signals representing the phase-to-phase measuring
loops (the ZREL output). This is to simplify the connection between PPL2PHIZ
and the distance protection.
The phase-to-phase measuring loops have actually nothing to do with phase
preference and are always enabled.
The ZREL output of the PPL2PHIZ function should be connected to the
RELCNDZx inputs of the High speed distance protection ZMFPDIS. Figure
shows how the information from PPL2PHIZ is affecting the interior of the distance
protection. As can be seen from the figure, the enabling signals for the six
measuring loops are in AND condition with the corresponding signals of the phase
selection, loop by loop. In other words, the phase selection inside the distance
protection has to detect the fault as well before a trip from the distance zones can
be achieved.
1MRK 506 369-UEN B
Line distance protection REL670 2.2 IEC
Application manual
248