Functions
2.3 Differential Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
105
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 of 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. This is also
applicable if the charging current compensation is switched on, since the compensation is not effective for
charge comparison. If there are inductances or transformers 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. magne-
tizing current (rush current) to be expected.
For protected transformers, set the value
I
N Transformer
/u
k Transformer
.
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 Arc-suppression coil. As the Arc-suppression 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.
Pickup value when switching on the charge comparison
If bushing transformers are used for a transformer in the protected line section, stray fluxes through the bushing
transformers may occur when reclosing after an external fault. These stray fluxes may cause a distortion of the
secondary current and an overfunction of the charge comparison.
If bushing transformers are used, the setting value of parameter
1235
I-DIF>>SWITCHON
should be 2 to 3
times the setting value of
I-DIFF>>
. The default setting of
I-DIF>>SWITCHON
corresponds to the default
setting of parameter
1233
I-DIFF>>
. In the default setting, this parameter is therefore ineffective.
Charging current compensation
The charging current compensation can only operate if this function has been set during configuring the func-
tional scope (Section 2.1.2) in address
149
charge I comp.
=
Enabled
. Also, the line data must have been
configured (Section 2.1.4.1). With more than two line ends particularly the parameter in address
1114
Tot.Line Length
has to be considered. If the unit of length in address
236
is changed for the total line length
in address
1114
, the line data have to be set again for the unit of length which has been changed. It is possible
to enter unrealistic data (very long line with extremely high capacitance). The charging current compensation
is in that case ineffective and provides restraint with a very high restraint current. This can also be seen from
the measured restraint values and from the output of an „effective-OFF“ indication.
In address
1221
Ic-comp.
you can determine whether the charging current compensation is to be switched
ON
or
OFF
. Please note that the parameter
I-DIFF>
in address
1210
must absolutely be increased to a value
of 2 to 3 times
I
cN
before the compensation is switched
OFF
, because otherwise a spurious trip might be the
result.
