Siemens siprotec SJ62 User Manual page 192

SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015
The FFM will pick up on a ground voltage V
ified under 5302 FUSE FAIL 3Vo and on a ground current IN which is smaller than
the threshold specified under 5303 FUSE FAIL RESID.
Pickup will take place on the specified values. A hysteresis is integrated for dropout,
of 105% where I or of 95% where V
N
the power system with weak infeed, the ground current caused by the fault might lie
below the pickup threshold of the Fuse Failure Monitor. Overfunctioning of the Fuse
Failure Monitor can, however, cause the feeder protection equipment to fail since it will
block all protective functions that use voltage signals. Such an overfunction of the FFM
is avoided by additionally checking the phase currents. If at least one phase currents
lies above the pickup threshold of 5303 FUSE FAIL RESID, it can be assumed that
the zero current created by a short-circuit would equally exceed this limit.
The following conditions hold to immediately detect a fault existing after activation of
the FFM: If a ground current I
rion was detected, a fault is assumed and the blocking by the Fuse Failure Monitor is
blocked for as long as the fault persists. If the voltage failure criterion applies for longer
than approx. 10 seconds, the blocking takes permanent effect. After the time has
elapsed it can be assumed that a Fuse Failure has actually occurred. The blocking is
lifted automatically 10 seconds after the voltage criterion has disappeared as a result
of the secondary circuit fault being cleared, and the entire protection function is re-
leased.
The following figure shows the logic diagram of the Fuse Failure Monitor.
Figure 2-61 Logic diagram of the Fuse Failure Monitor
which is bigger than the threshold spec-
N
. In case of a low-current asymmetrical fault in
N
occurs within 10 seconds after the Fuse-Failure crite-
N
2.11 Monitoring Functions
190