NOTE
When delivered from factory, the differential protection is switched OFF. The reason is that the protection
must not be in operation unless at least the connection group (of a transformer) and the matching factors
have been set before. Without proper settings, the device may show unexpected reactions (incl. tripping)!
Starpoint Conditioning
If there is a current transformer in the starpoint connection of an earthed transformer winding, i. e. between
starpoint and earth electrode, the starpoint current may be taken into consideration for calculations of the
differential protection. The earth-fault sensitivity is thus ensured.
If a starpoint is earthed but the earth current is not available for the measurement, the zero sequence current
is eliminated automatically in order to avoid false operation in case of an external earth fault; the following
parameters are then omitted. Equally, the parameters are not available if the respective transformer side has
no earthed starpoint in the protected zone. You have informed the device about the earthing conditions
during setting of the object properties (Section 2.1.4.2 General Power System Data under margin heading
“Object Data with Transformers”, addresses 313, 323, 333, 343 and/or 353 and Section 2.1.4.1 Topology of
the Protected Object under margin heading “Assignment of Further 1-phase Measuring Locations”).
The conclusion is: If the starpoint of a side of the protected power transformer is earthed and the starpoint
current is fed to the device (via a further 1-phase current input) you can, nevertheless, leave the default
setting for inclusion of the earth current unchanged in address 1211 DIFFw.IE1-MEAS for side 1 on YES.
This parameter can only be altered with DIGSI at Additional Settings. The same considerations apply to any
other or additional earthed sides:
•
1212 DIFFw.IE2-MEAS, for side 2 if earthed,
•
1213 DIFFw.IE3-MEAS, for side 3 if earthed,
•
1214 DIFFw.IE4-MEAS, for side 4 if earthed,
•
1215 DIFFw.IE5-MEAS, for side 5 if earthed.
During setting YES the corresponding earth current will be considered by the differential protection.
In auto-transformers the earth current flowing in the winding can be considered even if a complete three-
phase CT set has been installed as illustrated in Figure 2-6, where instead of measuring location Z3 also the
three phase currents can be connected to a three-phase measuring input of the device. The device then calcu-
lates the sum of the three currents and uses it as earth current. Set address 1216 DIFFw.IE3phMEAS to YES.
It is required to assign the respective three-phase measuring location to one side and to declare it as earth
winding (the side of the auto-connected winding facing the earth electrode). This parameter can only be
altered in DIGSI at Additional Settings.
Differential Current Monitoring
With
busbar protection or short-line protection differential current can be monitored. At address 1208 I-
DIFF> MON. the monitoring can be set to ON and OFF. Its use is only sensible if one can distinguish clearly
between operational error currents caused by missing transformer currents and fault currents caused by a
fault in the protected object.
The pickup value I-DIFF> MON. (address 1281) must be high enough to avoid a pickup caused by a transfor-
mation error of the current transformers and by minimum mismatching of different current transformers. On
the other hand, the pickup value must lie clearly below the differential protection (I-DIFF>, address 1221);
otherwise no differentiation between operational errors caused by missing secondary currents and fault
currents due to short-circuit in the protected object would be possible. The pickup value is referred to the
rated current of the protected object. Time delay T I-DIFF> MON. (address 1282) applies to the annuncia-
tion and blocking of the differential protection. This setting ensures that blocking with the presence of faults
(even of external ones) is avoided. The time delay usually amounts to a few seconds.
Feeder Current Guard
With
busbars and short lines a release of the trip command can be set if one of the incoming currents is
exceeded. The differential protection only trips if one of the measured currents exceeds the threshold I>
Functions
2.2 Differential Protection
116 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
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