Siemens siprotec 7SA6 User Manual page 298

Functions
Switching on to a
Dead Earth Fault
Phase Current
Stabilization
Inrush Stabilization
6-116
It is possible to determine with a setting which stage trips without delay following
closure onto a dead fault. The stages have the setting parameters ,!!!627)
7ULS (address ), ,!! 627)7ULS (address ), ,! 627)7ULS
(address ) and if required ,S 627)7ULS (address  ), which must
accordingly be set for each stage to either <HV or 1R. 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
instantaneous switch-onto-fault stage.
To avoid faulty pick up as a result of transient overcurrents, a time delay 627) 7LPH
'(/$< (address ) can be set. The presetting  is usually correct. In the case of
long cables, where large peak inrush currents can occur, a short delay may be useful.
This delay depends on how severe and how long the transient is, and which stages
are used for the switch-onto-fault tripping.
With the parameter 627) 2S 0RGH (address ) it is finally possible to determine
whether the fault direction must be checked (3,&.83',5(&7) or not (3,&.83),
before a switch-onto-fault tripping is generated. It is the direction setting for each stage
that applies for this direction check.
To avoid a faulty pick-up of the stages in the case of unsymmetrical load conditions or
different current transformer measuring errors in earth systems, the earth current
stages are stabilized by the phase currents: the pick up thresholds are increased as
the phase currents increase (refer also to Figure 6-65). By means of the setting in
address  ,SK67$% 6ORSH the preset value of 10 % for all stages can be
jointly changed for all stages. This setting can only be changed with DIGSI
"Additional 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
content 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
inhibited. This threshold value which is set in the address  ,PD[ ,QUXVK5HVW,
should be larger than the maximum expected inrush current (RMS value).
®
4 under
7SA6 Manual
C53000-G1176-C133-1