characteristic. In this case, the indication will be restricted to a pulse lasting for one or two power
system cycles.
The phase-selection element can, owing to the current change criteria, distinguish faults with
minimum influence from load and fault impedance. In other words, it is not restricted by a load
encroachment characteristic during the current change phase. This significantly improves
performance for remote phase-to-ground faults on heavily loaded lines. One exception, however,
are three-phase faults to which the load encroachment characteristic always has to be applied in
order to distinguish fault from load.
Phase-to-phase-ground faults (also called double ground faults) will practically always activate
phase-to-phase zone measurements. Measurement in two phase-to-ground loops at the same
time is associated with so-called simultaneous faults: two ground faults at the same time, one
each on the two circuits of a double line, or when the zero sequence current is relatively high due
to a source with low Z0/Z1 ratio. In these situations zone measurement will be released both for
the related phase-to-ground loops and the phase-to-phase loop simultaneously. On the other
hand, simultaneous faults closer to the remote bus will gradually take on the properties of a
phase-to-phase-ground fault and the function will eventually use phase-to-phase zone
measurements also here.
In cases where the fault current infeed is more or less completely of zero sequence nature (all
phase currents in phase), the measurement will be performed in the phase-to-ground loops only
for a phase-to-phase-ground fault.
ForcePE
PHSLxLy
MAX
IAMag
IBMag
ICMag
INReleasePE
INMag
2-phase Fault
detected by
Impedance/ current
based Phase
selection
250%
50%
PHSLy
AND
I3P
V3P
2-phase fault
I
N
/ I
max
AND
AND
OR
OR
a
b
a
b
a
b
a>b
a>b
a<b
AND
a
b
a<b
ANSI17000230-1-en.vsdx
ANSI17000230 V1 EN-US
Figure 149: Phase-selection logic
Figure
149 explains the release of two-phase faults (including simultaneous faults
as well as cross-country faults for high impedance grounded networks. This is not
valid for single-phase faults.
However, should it be desirable to use phase-to-ground (and only phase-to-ground) zone
measurement for phase-to-phase-ground faults, there is a setting
3I0Enable_PG
that can be
lowered from its excessive default value to the level above which phase-to-ground measurement
should be activated.
Section 7 1MRK 502 066-UUS B
Impedance protection
304
Technical manual
To Next Page
Loading...
