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Functions
Switching onto a
Dead Earth Fault
Phase Current
Stabilization
Inrush Stabilization
6-134
It is possible to determine with a setting which stage trips without delay following clo-
sure onto a dead fault. The stages have the setting parameters 3I0>>>SOTF-Trip
(address 3114 ), 3I0>> SOTF-Trip (address 3124 ), 3I0> SOTF-Trip (address
3134 ) and if required 3I0p SOTF-Trip (address 3149 ), which must accordingly be
set for each stage to either Yes or No . Selection of the most sensitive stage is usually
not reasonable as a solid short-circuit may be assumed following switching onto a
fault, whereas the most sensitive stage often also has to detect high resistance faults.
Transient pick-up of the selected stage, during energization of a healthy line, must be
avoided.
On the other hand, it does not matter if a selected stage may pick up due to inrush
conditions on transformers (see "Inrush Stabilization" below). The switch-onto-fault
tripping of a stage is blocked by the inrush stabilization even if it is set as instantane-
ous switch-onto-fault stage.
To avoid faulty pick up as a result of transient overcurrents, a time delay SOTF Time
DELAY (address 3173 ) can be set. The presetting 0 is usually correct. In the case of
long cables, where large peak inrush currents can occur, a short delay may be useful.
This delay depends on how severe and how long the transient is, and which stages
are used for the switch-onto-fault tripping.
With the parameter SOTF Op. Mode (address 3172 ) it is finally possible to determine
whether the fault direction must be checked ( PICKUP+DIRECT. ) or not ( PICKUP ), be-
fore a switch-onto-fault tripping is generated. It is the direction setting for each stage
that applies for this direction check.
To avoid a faulty pick-up of the stages in the case of unsymmetrical load conditions or
different current transformer measuring errors in earth systems, the earth current stag-
es are stabilized by the phase currents: the pick up thresholds are increased as the
phase currents increase (refer also to Figure 6-73). By means of the setting in address
3104A Iph-STAB. Slope the preset value of 10 % for all stages can be jointly
changed for all stages. This setting can only be modified with DIGSI
tional Settings ".
The inrush stabilization is only required if the device is applied to transformer feeders
or on lines that end on a transformer; in this case also only for such stages that have
a pick-up threshold below the inrush current and have a very short or zero delay. The
parameter 3I0>>>InrushBlk (address 3115 ), 3I0>> InrushBlk (address
3125 ), 3I0> InrushBlk (address 3135 ) and 3I0p InrushBlk (address 3150 )
may be set to Yes (inrush stabilization activated) or No (inrush stabilization disabled)
for each stage. If the inrush stabilization has been disabled for all stages, the following
parameters are of no consequence.
For the recognition of the inrush current, the portion of second harmonic current con-
tent referred to the fundamental current component can be set in address 3170 2nd
InrushRest . Above this threshold the inrush blocking is effective. The preset value
(15 %) should be sufficient in most cases. Lower values imply higher sensitivity of the
inrush blocking (smaller portion of second harmonic current results in blocking).
In applications on transformer feeders or lines that are terminated on transformers it
may be assumed that, if very large currents occur, a short circuit has occurred in front
of the transformer. In the event of such large currents, the inrush stabilization is inhib-
ited. This threshold value which is set in the address 3171 Imax InrushRest ,
should be larger than the maximum expected inrush current (RMS value).
®
4 under " Addi-
7SA6 Manual
C53000-G1176-C156-2
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