Siemens siprotec 7SD5 User Manual page 227

3IoMin Teleprot (address 3105) must be set to avoid non-selective tripping during
through-fault earth current measurement. For further information refer to Section 2.9,
margin heading „Earth Fault Protection Prerequisites".
Switching onto an It is possible to determine with a setting which stage trips without delay following
closure onto a dead fault. The parameters 3I0>>>SOTF-Trip (address 3114),
Earth Fault
3I0>> SOTF-Trip (address 3124), 3I0> SOTF-Trip (address 3134) and, if nec-
essary, 3I0p SOTF-Trip (address 3149) are available for the stages which can be
set to YES or NO for each stage. 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. It is
important to avoid that the selected stage picks up in a transient way during line ener-
gization.
On the other hand, it does not matter if a selected stage may pick up due to inrush
conditions on transformers. The switch-onto-fault tripping of a stage is blocked by the
inrush restraint even if it is set as instantaneous switch-onto-fault stage for manual clo-
sure.
To avoid a spurious pickup due to transient overcurrents, the delay SOTF Time
DELAY (address 3173) can be set. Usually, the default setting 0 can be retained. In
the case of long cables, where large peak inrush currents can occur, a short delay may
be useful. The time delay depends on the severity and duration of the transient over-
currents as well as on which stages were selected for the fast switch onto fault clear-
ance.
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),
before a switch-onto-fault tripping is generated. It is the direction setting for each stage
that applies for this direction check.
Phase Current Re- To avoid a spurious pickup of the stages in the case of asymmetrical load conditions
straint or varying current transformer measuring errors in earth systems, the earth current
stages are restrained by the phase currents: as the phase currents increase, the
pickup thresholds are increased. By means of the setting in address 3104 Iph-
STAB. Slope the preset value of 10 % for all stages can be jointly changed for all
stages. This setting is only possible via DIGSI
Inrush Restraint The inrush restraint 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
pickup threshold below the inrush current and have a very short or zero delay. The pa-
rameters 3I0>>>InrushBlk (address 3115), 3I0>> InrushBlk (address 3125),
3I0> InrushBlk (address 3135) and 3I0p InrushBlk (address 3150) can be
set to YES (inrush restraint active) or NO (inrush restraint inactive) for each stage. If the
inrush restraint 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
content 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 restraint is inhibited.
7SD5 Manual
C53000-G1176-C169-1
2.8 Earth Fault Protection in Earthed Systems (optional)
®
at Additional Settings.
227