2.7 Earth fault overcurrent protection in earthed systems (optional)
171
7SA522 Manual
C53000-G1176-C155-3
,!!7HOHS%, for stage 3I
0
>>, address ,!!!7HOHS%, for stage
3I
0
>>>, address ,S7HOHS%, for stage 3I
0P
(if used).
If the echo function is used in conjunction with the teleprotection scheme, or if the
weak-infeed tripping function should be used, the additional teleprotection stage
,R0LQ7HOHSURW (address ) must be set to avoid non-selective tripping during
through-fault earth current measurement. For further information refer to Section 2.8,
margin heading “Earth Fault Protection Prerequisites”.
Switching onto an
Earth Fault
It is possible to determine with a setting which stage trips without delay following
closure onto a dead fault. The parameters ,!!!627)7ULS (address
), ,!!627)7ULS (address ), ,!627)7ULS (address ) and
if necessary ,S627)7ULS (address ) are available for the stages, which
can be set to <(6 or 12 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
energization.
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 stabilization even if it is set as instantaneous switch-onto-fault stage for manual
closure.
To avoid a spurious pickup due to transient overcurrents, the delay 627)7LPH
'(/$< (address ) can be set. Usually, the default setting 0 can be retained. In
the case of long cables, where large peak inrush currents can occur, a short delay may
be useful. The time delay depends on the severity and duration of the transient over-
currents as well as on which stages were selected for the fast switch onto fault clear-
ance.
With the parameter 627)2S0RGH (address ) it is finally possible to determine
whether the fault direction must be checked (3,&.83',5(&7) or not (3,&.83),
before a switch-onto-fault tripping is generated. It is the direction setting for each stage
that applies for this direction check.
Phase Current
Stabilization
To avoid a faulty pickup of the stages in the case of asymmetrical load conditions or
varying current transformer measuring errors in earthed systems, the earth current
stages are stabilized by the phase currents: As the phase currents increase, the
pickup thresholds are increased. By means of the setting in address ,SK
67$%6ORSH the preset value of 10 % for all stages can be jointly changed for all
stages. This setting is only possible via DIGSI
®
at "Additional Settings".
Inrush
Stabilization
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 pickup threshold below the inrush current and have a very short or zero delay. The
parameters ,!!!,QUXVK%ON (address ), ,!!,QUXVK%ON (address
), ,!,QUXVK%ON (address ) and ,S,QUXVK%ON (address )
can be set to <(6 (inrush stabilization active) or 12 (inrush stabilization inactive) 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).
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