Purpose of the Fuse Failure Monitor
In case of a measuring voltage failure caused by a fault or a broken wire in the voltage transformer secondary
system, a zero voltage can be simulated to individual measuring loops.
The displacement voltage element of the (sensitive) ground fault detection, the directional overcurrent protec-
tion and the undervoltage protection can thereby acquire incorrect measuring results.
The blocking of this function by the FFM is configurable.
The FFM can become effective in grounded as well as in isolated systems provided that the connection mode
Van, Vbn, Vcn or Vab, Vbc, VGnd was set. Of course, the miniature circuit breaker and FFM can also be
used for the detection of a measuring voltage failure at the same time.
The displacement voltage element of the (sensitive) ground fault detection, the time overcurrent protection
ground and the undervoltage protection can thereby obtain incorrect measuring results.
Mode of Operation – Grounded System
The device is informed of the application of the FFM in a grounded system via address 5301 FUSE FAIL
MON. Solid grounded.
NOTE
On systems where the 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 operating mode in a grounded system is illustrated in Figure 2-59. Depending on the
configuration and MLFB, the FFM operates with measured or calculated values V
N
or Ι
N
. If zero sequence
voltage occurs without a ground fault current being registered simultaneously, then there is an asymmetrical
fault in the secondary circuit of the voltage transformer.
The displacement voltage element of the (sensitive) ground fault detection, the directional overcurrent protec-
tion (phase and ground function) 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 set limit value under address5302 FUSE FAIL
3Vo and if the ground current IN lies below the set limit value under address 5303 FUSE FAIL RESID.
Pickup occurs in accordance with the configured values. A hysteresis of 105% for the dropout is integrated for
Ι
N
or 95% for V
N
. In case of low-current asymmetrical faults in systems with week infeed, the ground current
caused by the fault could lie below the pickup threshold of the Fuse Failure Monitor. Overfunctioning of the
Fuse Failure Monitor can, however, cause the feeder protection device to underfunction since all protection
functions that use voltage signals are blocked. In order to prevent 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 fault will equally exceed this threshold.
In order to immediately detect an existing fault after switching in, the following applies: If a ground current Ι
N
is detected within 10 seconds after recognition of the Fuse Failure criterion, the protection assumes a fault
and removes the blocking by the Fuse Failure Monitor for the duration of the fault. If, on the other hand, the
voltage failure criterion is present for longer than approx. 10 s, the blocking is permanently activated. After
this time has elapsed, it can be assumed that a fuse failure has actually occurred. Only 10 s after the voltage
criterion has been removed by correction of the secondary circuit failure, will the blocking automatically reset,
thereby releasing the blocked protection functions.
The generation of the internal signal “Alarm FFM” for the operation in isolated systems is shown in
Figure 2-60.
Functions
2.11 Monitoring Functions
SIPROTEC 4, 7SJ80, Manual 159
E50417-G1140-C343-A8, Edition 12.2017
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