Siemens siprotec 7SA522 User Manual page 256

Functions
Phase Current
Stabilization
Inrush Stabilization
6-108
fault, whereas the most sensitive stage often also has to detect high resistance faults.
Transient pick-up of the selected stage, during line energization, 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 627) 7LPH
'(/$< (address ) can be set. The presetting  is usually correct. In the case of
long cables, where large peak inrush currents can occur, a short delay may be useful.
This delay depends on how severe and how long the transient is, and which stages
are used for the switch-onto-fault tripping.
With the parameter 627) 2S 0RGH (address ) it is finally possible to determine
whether the fault direction must be checked (3,&.83',5(&7) or not (3,&.83), be-
fore a switch-onto-fault tripping is generated. It is the direction setting for each stage
that applies for this direction check.
To avoid a faulty pick-up of the stages in the case of unsymmetrical load conditions or
varying current transformer measuring errors in earth systems, the earth current stag-
es are stabilized by the phase currents: the pick up thresholds are increased as the
phase currents increase (refer also to Figure 6-60). By means of the setting in address
 ,SK67$% 6ORSH 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 ,!!!,QUXVK%ON (address ), ,!! ,QUXVK%ON (address
), ,! ,QUXVK%ON (address ) and ,S ,QUXVK%ON (address )
may be set to <HV (inrush stabilization activated) or 1R (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  QG
,QUXVK5HVW. 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  ,PD[ ,QUXVK5HVW,
should be larger than the maximum expected inrush current (RMS value).
®
4 under "Addi-
7SA522 Manual
C53000-G1176-C119-2