Functions
2.5 Inrush Restraint
SIPROTEC, 7SD80, Manual
E50417-G1140-C474-A1, Release date 09.2011
98
2.5
Inrush Restraint
2.5.1
Description
If the protected zone of the device reaches beyond a transformer, a high inrush current must be anticipated
when switching on the transformer. This current flows into the protected zone, but does not leave it again.
The inrush current can amount to a multiple of the rated current and is characterized by a considerable 2nd
harmonic content (double rated frequency) which is practically absent during a short circuit. If the second har-
monic content in the differential current exceeds a selectable threshold, tripping is blocked.
The inrush restraint has an upper limit: It no longer takes effect when a (configurable) current value is sur-
passed since, in this case, it can only be an internal high-current fault.
Figure 2-32 shows a simplified logic diagram. The condition for the inrush current detection is examined in each
device in which this function has been activated. The blocking condition is transmitted to the other device so
that it is effective at both ends of the protected object.
Figure 2-32
Logic diagram of the inrush restraint for one phase
Since the inrush restraint operates individually for each phase, the protection is fully operative when the trans-
former is switched onto a single-phase fault, in which case it is possible for an inrush current to flow through
one of the undisturbed phases. It is, however, also possible to set the protection in such a way that when the
permissible harmonic content in the current of only one single phase is exceeded, not only the phase with the
inrush current but also the remaining phases of the differential stage are blocked. This cross-block function can
be limited to a selectable duration. Figure 2-33 shows the logic diagram.
The cross-block function also affects both devices since it not only extends the inrush current detection to all
three phases but also sends it to the other device via the communication link.
