Functions
2.10 Monitoring Functions
SIPROTEC, 7SK80, Manual
E50417-G1140-C344-A4, Release date 08.2010
162
Mode of Operation - Grounded System
The device is informed of the application of the FFM in the grounded system via address 5301 FUSE FAIL
MON. Solid grounded.
Note
On systems where ground fault current is very small or absent (e.g. ungrounded supply transformers), fuse
failure monitoring must be disabled or set to Coil.gnd./isol..
The logic diagram on the mode of operation in a grounded system is illustrated in Figure 2-49. Depending on
the configuration and MLFB, the FFM operates with measured or calculated values V
N
or I
N
. If a zero sequence
voltage occurs without a ground fault current being registered simultaneously, this suggests an asymmetrical
fault in the secondary voltage transformer circuit.
The displacement voltage element of the sensitive ground fault detection, the directional time overcurrent pro-
tection (Ground) and the undervoltage protection are blocked if parameter 5310 BLOCK PROT. is set to YES.
The FFM picks up if the ground voltage V
N
is higher than the limit value set at 5302 FUSE FAIL 3Vo and if
the ground current IN is smaller than the limit value set at 5303 FUSE FAIL RESID.
Pickup occurs at the configured values. A hysteresis for the dropout of 105% is integrated for I
N
or 95% for V
N
.
In the case of a low-current asymmetrical fault in a system with week infeed, the ground current caused by the
fault could lie below the pickup threshold of the FFM. An overfunctioning of the FFM can, however, cause the
feeder protection device to underfunction since all protection functions that use voltage signals are blocked. In
order to prevent such an overfunctioning of the FFM, the phase currents are also checked. If at least one phase
current lies above the pickup threshold of 5303 FUSE FAIL RESID, it can be assumed that the zero current
created by a short-circuit equally exceeds this threshold.
In order to immediately detect an existing fault after connection, the following applies: If a ground current I
N
larger than the pickup threshold of 5303FUSE FAIL RESID is detected within 10 seconds after recognition of
the fuse failure criterion, the protection assumes a short-circuit and removes the blocking by the FFM for the
duration of the fault. Whereas if the voltage failure criterion is present for more than about 10 seconds, the
blocking is permanently active. After this time has elapsed, it can be assumed that a fuse failure has actually
occurred. Only 10 seconds after the voltage criterion was removed by correction of the secondary circuit failure,
is the blocking automatically reset, thereby releasing the blocked protection functions.
The generation of the internal signal „Alarm FFM isol. N.“ for the mode of operation in an isolated system is
illustrated in Figure 2-50.
To Next Page
Loading...
