2.2 Differential Protection
109
7UT613/63x Manual
C53000-G1176-C160-2
Fast Unrestrained
Trip with High-
Current Faults
High-current faults in the protected zone may be cleared instantaneously without
regard to the restraint currents when the current amplitude excludes an external fault.
If the protected object has a high direct impedance (transformers, generators, series
reactors), a threshold can be found which can never be exceeded by a through-fault
current. This threshold (primary) is, for example, for a power transformer.
The differential protection of the 7UT613/63x 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 instantaneous value
stage operates only above twice the set threshold.
Increase of the
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 differ-
ent 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 over-
functioning when a non-energised protection object is switched in. As soon as the re-
straint 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 (figure
2-22).
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 I
diff
/I
stab
close to the
fault characteristic (≥ 80 % of the fault characteristic slope) cause tripping even before
the time T START MAX has lapsed.
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