Siemens SIPROTEC 7UT613 series Manual page 131

Differential Current
Monitoring
Feeder Current
Guard
Trip Characteristic
Differential Current
7UT613/63x Manual
C53000-G1176-C160-2
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 transformation 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 annunciation 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.
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 mea-
sured currents exceeds the threshold I> CURR. GUARD (address 1210). The pickup
value is referred to the rated current of the respective side. With setting 0.00 I/InS (pre-
setting) this release criterion will not be used.
If the feeder current guard is set (i. e. to a value of > 0), the differential protection will
not trip before the release criterion is given. This is also the case if, in conjunction with
very high differential currents, the extremely fast instantaneous value scheme has de-
tected the fault already after a few milliseconds.
The parameters of the tripping characteristic are set in addresses 1221 to 1265.
Figure 2-49 illustrates the meaning of the different settings. The numbers at the differ-
ent branches of the characteristic signify the addresses of the settings.
I-DIFF> (address 1221) is the pickup value of the differential current. This is the total
fault current into the protected object, regardless of the way this is distributed between
the sides. The pickup value is referred to the rated current of the protected object. You
may select a high sensitivity (small pickup value) for transformers (presetting
0.2 · I
). With reactors, generators and motors the sensitivity can be set even
N Obj
higher, provided that the current transformer sets are of similar design. A higher value
(above rated current) should be selected for lines and busbars. Higher measuring tol-
erances must be expected if the rated currents of the current transformers differ ex-
tensively from the rated current of the protected object, or if there are multiple measur-
ing locations.
In case of busbars and short lines, the through-flowing current can increase substan-
tially, depending on the system. The unstable I-DIFF>> stage could trip. In such
cases I-DIFF>> should be set to ∞.
2.2 Differential Protection
131