Siemens SIPROTEC 4 7UT6 Series Manual page 117

CURR. GUARD (address 1210). The pickup value is referred to the rated current of the respective side. With
setting 0.00 (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 detected the fault already after a few milliseconds.
Trip Characteristic Differential Current
The parameters of the tripping characteristic are set in addresses 1221 to 1263.
meaning of the different settings. The numbers at the different 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 trans-
formers (presetting 0.2 · Ι
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 tolerances must be expected if the rated currents of the
current transformers differ extensively from the rated current of the protected object, or if there are multiple
measuring locations.
Besides the threshold I-DIFF> is a second threshold available. In case of exceeding this threshold I-DIFF>>
(address 1231) is independent of the value of the restraint current and recognized independently of a possible
inrush stabilization on a tripping (not stabilized instantaneous stage). This stage must be set greater than I-
DIFF>. For protection objects with high longitudinal impedance (Transformer, Generator, Reactor) can find a
current value of a flowing through the short circuit current is not exceeded. In a transformer e.g., the
(primary) value:
NOTE
i
i
For setting the correct threshold of I-DIFF>>, the short circuit impedance U
external fault current Ι
object must be observed.
[diff-stromstfehler-021026-rei, 1, en_GB]
In case of busbars and short lines, the through-flowing current can increase substantially, depending on the
system. The unstable I-DIFF>> stage could trip. In such cases I-DIFF>> should be set to ∞.
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
). With reactors, generators and motors the sensitivity can be set even higher,
N Obj
'', the maximum inrush current and the short circuit impedance of the protected
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Functions
2.2 Differential Protection
Figure 2-49 shows the
'' of the grid, the maximum
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117