[diff-ausloesekennl-020926-rei, 1, en_GB]
Figure 2-21 Tripping characteristic of the differential protection and fault characteristic
Add-on Restraint during External Faults
Saturation of the current transformers caused by high fault currents and/or long system time constants are
uncritical for internal faults (fault in the protected zone), since the measured value deformation is found in the
differential current as well in the restraint current, to the same extent. The fault characteristic as illustrated in
Figure 2-21 also applies in principle in this case. Of course, the fundamental wave of the current must exceed
at least the pickup threshold (branch a).
During an external fault which produces a high through-flowing fault current causing current transformer
saturation, a considerable differential current can be simulated, especially when the degree of saturation is
different at the two sides. If the quantities Ι
diff
/Ι
stab
result in an operating point which lies in the trip area of the
operating characteristic, trip signal would be the consequence if there were no special measures.
7UT6x provides a saturation indicator which detects such phenomena and initiates add-on restraint (stabilisa-
tion) measures. The saturation indicator considers the dynamic behaviour of the differential and restraint
quantity
The dotted line in Figure 2-21 shows the instantaneous currents during an external fault with transformer
saturation on one side.
Immediately after the fault (A), the short-circuit currents rise strongly, causing a correspondingly high
restraint current (2 × through-flowing current). At the instant of CT saturation (B), a differential quantity is
produced and the restraint quantity is reduced. In consequence, the operating point Ι
diff
/Ι
stab
may move into
the tripping area (C).
In contrast, the operating point moves immediately along the fault characteristic (D) when an internal fault
occurs since the restraint (stabilisation) current will barely be higher than the differential current.
Current transformer saturation during external faults is detected by the high initial restraining current which
moves the operating point briefly into the add-on restraint area. The saturation indicator makes its decision
within the first quarter cycle after fault inception. When an external fault is detected, the differential protec-
tion is blocked for a selectable time. This blocking is cancelled as soon as the operation point Ι
diff
/Ι
stab
is
stationary (i.e. throughout at least one cycle) within the tripping zone near the fault characteristic (≥ 80 % of
the fault characteristic slope). This allows consequential faults in the protected area to be quickly recognised
even after an external fault involving current transformer saturation.
The add-on restraint acts individually per phase. It can be determined by a setting parameter whether only the
phase with detected external fault is blocked when this restraint criterion is fulfilled or also the other phases of
the differential stage.
A further stabilisation (restraint) comes into effect when differential secondary currents are simulated by
different transient behaviour of the current transformer sets. This differential current is caused by different DC
time constants in the secondary circuits during through-current conditions, i.e. the equal primary DC compo-
Functions
2.2 Differential Protection
SIPROTEC 4, 7UT6x, Manual 95
C53000-G1176-C230-5, Edition 09.2016
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