Table of Contents
Advertisement
2.8.1.5 Measurement Voltage Failure Detection
Requirements
The measurement voltage failure detection function, referred to as „Fuse Failure Monitor" (FFM), only operates
under the following condition:
• Three phase-to-ground voltages are connected; with phase-to-phase voltages and V
nection, the function is disabled.
Purpose of the Fuse Failure Monitor
In the event of a measuring voltage failure due to a short circuit fault or a broken conductor in the voltage trans-
former secondary circuit, certain measuring loops may mistakenly see a voltage of zero. Directional overcurrent
protection and undervoltage protection may thereby acquire incorrect measuring results.
Of course, supervision of the miniature circuit breaker and the Fuse Failure Monitor can be used at the same
time.
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 for the mode of operation in a grounded system is illustrated in Figure 2-36. Depending on
the configuration and MLFB, the FFM operates with measured or calculated values V
occurs without a ground fault current being registered simultaneously, this suggests an asymmetrical fault in
the secondary voltage transformer circuit. Directional overcurrent protection (phase and ground function) and
undervoltage protection are blocked if parameter 5310 BLOCK PROT. is set to YES.
The FFM picks up if the ground voltage V
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
I
or 95% for V
N
N
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 I
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 re-
leasing the blocked protection functions.
The generation of an internal signal „Alarm FFM isol. N." for the mode of operation in an isolated system is
illustrated in Figure 2-37.
SIPROTEC, 7SC80, Manual
E50417-G1140-C486-A1, Release date 12.2011
is higher than the set limit value under address 5302 FUSE FAIL
N
. In case of low-current asymmetrical faults in systems with week infeed, the ground current
Functions
2.8 Monitoring Functions
or single-phase con-
N
or I
. If a zero voltage
N
N
is
N
125
Advertisement
Table of Contents
