Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
78
Cross Blocking
Since inrush restraint operates individually for each phase, protection is ideal where a power transformer is en-
ergized into a single-phase fault and inrush currents are detected on a different healthy phase. However, the
protection feature can be configured to allow that not only this phase element but also the remaining elements
(including ground) are blocked (the so-called CROSS BLOCK function, address 2203) if the permissible har-
monic component of the current is exceeded for only one phase.
Please take into consideration that inrush currents flowing in the ground path will not
cross-block tripping by
the phase elements.
Cross blocking is reset if there is no more inrush in any phase. Furthermore, the cross blocking function may
also be limited to a particular time interval (address 2204 CROSS BLK TIMER). After expiry of this time interval,
the cross blocking function will be disabled, even if inrush current is still present.
The inrush restraint has an upper limit: Above this (via adjustable parameter 2205 I Max) current blocking is
suppressed since a high-current fault is assumed in this case.
The following figure shows the inrush restraint influence on the time overcurrent elements including cross-
blocking.
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