Siemens SIPROTEC 4 7UT6 Series Manual page 97

The differential protection of the 7UT6x provides such an unstabilised high-current trip stage. This stage can
operate even when, for example, a considerable second harmonic is present in the differential current caused
by current transformer saturation by a DC component in the fault current, which could be interpreted by the
inrush restraint function as an inrush current.
Fast tripping uses both the fundamental component of the differential current as well as instantaneous values.
Instantaneous value processing ensures fast tripping even if the current fundamental component was strongly
attenuated by current transformer saturation. Due to the possible DC offset after fault inception, the instanta-
neous value stage operates only above 2 · √2 · set threshold.
Increase of Pickup Value on Startup
The increase of pickup value is especially suited for motors. In contrast to the inrush current of transformers
the inrush current of motors is a traversing current. Differential currents, however, can emerge if current
transformers still contain different remanent magnetisation before energise. Therefore, the transformers are
energised from different operation points of their hysteresis. Although differential currents are usually small,
they can be harmful if the differential protection is set very sensitive.
An increase of the pickup value on startup provides additional security against overfunctioning when a none-
nergised protection object is switched in. As soon as the restraint current of one phase has undershot a
settable value I-REST. STARTUP, the increase of the pickup value is activated. As the restraint current is
twice the through-flowing current in normal operation, its undershooting of that threshold is a criterion for
detecting that the protected object is not energised. The pickup value I-DIFF> and the other branches of the
IDiff> stage are now increased by a settable factor
The return of the restraint current indicates the startup. After a settable time T START MAX the increase of
the characteristic is undone. Current ratios Ι
teristic slope) cause tripping even before the time T START MAX has lapsed.
[diff-ansprechwanlauf-020926-rei, 1, en_GB]
Figure 2-22
Tripping Characteristic
Figure 2-23 illustrates the complete tripping characteristic of the 7UT6x. The characteristic branch a repre-
sents the sensitivity threshold of the differential protection (setting I-DIFF>) and considers constant error
currents such as magnetising currents.
Branch b considers current-proportional errors which may result from transformation errors of the main CTs or
the input CTs of the device, or which for example may be caused by mismatching or by the influence of tap
changers in transformers with voltage control.
For high currents which may give rise to current transformer saturation, characteristic branch c provides for
additional restraint.
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
Increase of pickup value of the stage on startup
(Figure 2-22).
/Ι close to the fault characteristic (≥ 80 % of the fault charac-
diff stab
Functions
2.2 Differential Protection
97