Siemens SIPROTEC 4 7UT6 Series Manual page 144

Functions
2.4 Time Overcurrent Protection for Phase and Residual Currents
protection. After detection of inrush currents above a pickup value, special inrush signals are generated. These
signals also initiate fault annunciations and start the assigned trip delay time. If inrush current is still detected
after expiration of the delay time, an annunciation is output only reporting that time elapsed. Tripping,
however, is suppressed.
The inrush current is characterised by a considerable 2nd harmonic content (double rated frequency) which is
practically absent in the case of a short-circuit. If the second harmonic content of a phase current exceeds a
selectable threshold, trip is blocked for this phase. The same applies to the zero sequence current.
The inrush restraint has an upper limit: if a certain (adjustable) current value is exceeded, it will no longer be
effective, since there must be an internal current-intensive short-circuit. The lower limit is the operating limit
of the harmonic filter (0.1 Ι
Figure 2-74 shows a simplified logic diagram.
[logikdia-einschaltstabilisierung-bsp-phasen-121102-st, 1, en_GB]
Figure 2-74
Since the harmonic restraint operates individually per phase, the protection is fully operative even when e.g.
the transformer is switched onto a single-phase fault, whereby inrush currents may possibly be present in one
of the healthy phases. However, it is also possible to set the protection such that not only the phase with
inrush current exhibiting harmonic content in excess of the permissible value is blocked but also the other
phases of the associated stage are blocked (so called "cross-block function"). This cross-block can be limited to
a selectable duration.
Cross-block refers only to the three phases. Phase inrush currents do not block the residual current stages nor
vice versa.
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Logic diagram of the inrush restraint feature — example for phase currents (simplified)
Figure 2-75 shows the logic diagram of this function.
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016