Siemens SIPROTEC 7SD610 Manual page 74

Functions
2.3 Differential Protection
Pickup value during switch-on
When switching on long, unloaded cables, overhead lines and arc-compensated lines, pronounced higher-fre-
quency 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 active whenever a device has recognized the con-
nection 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 sensitivity. A setting to three to four times the steady-state charging current usually ensures 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 Restraint").
The pickup thresholds are checked during commissioning. For further information please refer to the chapter
Mounting and Commissioning.
Delays
In special cases of application it may be useful to delay the tripping of the differential protection with an addi-
tional 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 at Additional Set-
tings.
If the differential protection is applied to an isolated or resonant-earthed network, it must be ensured that trip-
ping is avoided due to the transient oscillations of a single earth fault. With address 1218 T3I0 1PHAS pickup
on a single fault is therefore delayed for 0.04 s (default setting). For large resonant-earthed systems the time
delay should be increased. By setting the address to ∞, the single-phase pickup is suppressed entirely.
In an earthed system, T3I0 1PHAS shall be set to 0.00 s. This parameter can only be altered in DIGSI at Ad-
ditional 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.
Pickup value charge comparison stage
The pickup threshold of the charge comparison stage is set in address 1233 I-DIFF>>. The RMS value of the
current is decisive. The conversion into charge value is carried out by the device itself.
In most cases, it is adequate to set the values to approx. 100 % to 200 % of the operational nominal current.
Please also consider that the setting is related to the operational nominal values that must be equal on the
pimary side at all ends of the protected object.
Since this stage reacts very quickly, a pickup of capacitive charging currents (for lines) and inductive magne-
tizing currents (for transformers or shunt reactors) must be excluded – also for switching processes. If induc-
tances or transformers are located in the protected area (between the current transformers of the differential
protection), the parameters must be set to a value higher than the max. magnetizing current (rush current) to
be expected.
For protected transformers, set the value I
In resonant-earthed systems, the value of the non-compensated system earth fault current should not be un-
dershot as well. This value derives from the capacitive earth fault current of the entire network without consid-
ering the Petersen coil. As the Petersen coil serves to compensate nearly the total earth fault current, its
nominal current can be approximately used as a basis.
The pickup thresholds are finally checked during commissioning. Further information can be found in chapter
Installation and Commissioning.
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N Transformer k Transformer
C53000-G1176-C145-6, Release date 02.2011
SIPROTEC, 7SD610, Manual