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2 Functions
2.1.2.1 Setting information, Margin heading „Power Transformer with Voltage Regula-
tion".
Pickup value
When switching on long, unloaded cables, overhead lines and arc-compensated lines,
during switch-on
pronounced higher-frequency transient reactions may take place. These peaks are
considerably damped by means of a digital filter in the differential protection. A pickup
value I-DIF>SWITCH ON (address 1213) can be set to reliably prevent single-sided
pickup of the protection. This pickup value is always active when a device has recog-
nized the connection of a dead line at its end. For the duration of the seal-in time SI
Time all Cl. which was set in the general protection data at address 1132 (Section
2.1.4.1) all devices are then switched over to this particular pickup value. A setting to
three to four times the steady-state charging current ensures usually the stability of the
protection during switch-on of the line. For switch-on of a transformer or shunt reactor,
an inrush restraint is incorporated (see below under margin heading „Inrush Re-
straint").
Final checks will be carried out during commissioning. Further information can be
found in chapter Installation and Commissioning.
Delays
In special cases of application it may be useful to delay the tripping of the differential
protection with an additional timer, e.g. in case of reverse interlocking. The delay time
T-DELAY I-DIFF> (address 1217) is only started upon detection of an internal fault.
This parameter can only be altered with DIGSI under Additional Settings.
If the differential protection is applied to an isolated or resonant-earthed network, it
must be ensured that tripping is avoided during the transient oscillations of a single
earth fault. With address 1218 T3I0 1PHAS the pickup to a single fault is therefore
delayed for 0.04 s (presetting). For large resonant-earthed systems the time delay
should be increased. By setting the address to ∞ the single-phase tripping is totally
suppressed.
In an earthed system, T3I0 1PHAS shall be set to 0.00 s. This parameter can only be
altered in DIGSI at Additional Settings.
If it is desired that a TRIP command is generated in the event of an internal fault only
if simultaneously the current of the local line end has exceeded a specific quantity,
then this current threshold can be set for enabling the differential protection TRIP at
address 1219 I> RELEASE DIFF. This parameter can only be altered in DIGSI at
Additional Settings.
The pickup threshold of the charge comparison stage is set in address 1233 I-
Pickup value
DIFF>>. The RMS value of the current is decisive. The conversion into charge value
charge comparison
stage
is carried out by the device itself.
Setting near the operational nominal current is adequate in most cases. Please also
remember that the setting is related to the operational nominal values that must be
equal (primary) at all ends of the protected object.
Since this stage reacts very fast, a pickup of capacitive charging currents (for lines)
and inductive magnetising currents (for transformers or shunt reactors) — also for
switch-on condition — must be excluded.
In resonant-earthed systems also the value of the non-compensated system earth
fault current may not be undershot. It derives from the total capacitive earth fault
current without considering the Petersen coil. As the Petersen coil serves to compen-
sate nearly the total earth fault current, its rated current can be taken as a base.
For transformers set the value I
72
/u
.
N Trafo
k Trafo
7SD610 Manual
C53000-G1176-C145-4
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