Functions
2.2 Overcurrent Protection 50, 51, 50N, 51N
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008
59
Since quantitative analysis of the harmonic components cannot be completed until a full line period has been
measured, pickup will generally be blocked by then. Therefore, assuming the inrush restraint feature is en-
abled, a pickup message will be delayed by a full line period if no closing process is present. On the other hand,
trip delay times of the time overcurrent protection feature are started immediately even with the inrush restraint
being enabled. Time delays continue running with inrush currents present. If inrush blocking drops out after the
time delay has elapsed, tripping will occur immediately. Therefore, utilization of the inrush restraint feature will
not result in any additional tripping delays. If a relay element drops out during inrush blocking, the associated
time delay will reset.
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.