ABB RELION RET670, Applications Manual

Page: 504 / 1016

a residual current of the same magnitude. This residual current may be used as an
indication of a cross-country fault.
IC=IG
IA=IG
en06000553_ansi.vsd
ANSI06000553 V1 EN-US
Figure 251: The currents in the phases at a double ground fault
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
Section 8
1MRK 504 163-UUS A
Impedance protection
498 Transformer protection RET670 2.2 ANSI
Application manual