2.3 Differential Protection
99
7SD5 Manual
C53000-G1176-C169-1
a single-phase pickup can be blocked for a short time in order to bridge the transient
oscillations on occurrence of a single earth fault in a resonant-earthed system.
The signals processed in this way are linked by the tripping logic of the device to the
output signals
„Diff. Gen. TRIP“
,
„Diff TRIP 1p L1“
,
„Diff TRIP 1p L2“
,
„Diff TRIP 1p L3“
,
„Diff TRIP L123“
. The single-pole information implies that
tripping will take place single-pole only. The actual generation of the commands for the
tripping (output) relay is executed within the tripping logic of the entire device (see
Section 2.23.1).
Figure 2-24
Tripping logic of the differential protection
2.3.2
Setting Notes
General
The differential protection can be switched
ON
or
OFF
in address
1201
STATE OF
DIFF.
. If a single device is switched off at any end of a protective object, the calcula-
tion of measured values becomes impossible. The entire differential protection of all
ends is then blocked; if the distance protection is available and configured, it repre-
sents the main protection function.
Pickup Value of the
Differential Current
The current sensitivity is set with address
1210
I-DIFF>
. It is determined by the
entire current flowing into a protected zone in case of a fault. This is the total fault
current regardless of how it is distributed between the ends of the protected object.
If charging current compensation has been switched
ON
at address
1221
Ic-comp.
=
ON
, the pickup value
I-DIFF>
can be set to 1 ·
I
cN
. Thus the residual error of the
charging current compensation is considered.
Without charging current compensation (address
1221
Ic-comp.
=
OFF
), this pickup
value must be set to a value that is higher than the total steady-state shunt current of
the protected object. For cables and long overhead lines, the charging current is to be
considered in particular.
The charging current is calculated from the operational capacitance:
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