Siemens SIPROTEC 4 7UT6 Series Manual page 264

Functions
2.19 Monitoring Functions
Asymmetrical measuring voltage failure "Fuse Failure Monitor"
In the event of measured voltage failure due to a short-circuit or a broken conductor in the voltage trans-
former secondary circuit, protection and monitoring functions whose operating principle is based on under-
shooting the measured voltage can cause faulty pickup, which would lead to a time-delayed spurious tripping.
In 7UT6x, this concerns the forward active power supervision P<, the undervoltage protection and the configu-
rable flexible protection functions.
If, for example, fuses are used instead of a secondary miniature circuit breaker with correspondingly
connected auxiliary contacts, then the fuse failure monitoring can become active. Of course, the VT miniature
circuit breaker and the fuse failure monitor can be used at the same time.
The asymmetrical measured voltage failure is characterised by its voltage asymmetrical with simultaneous
current symmetry.
failure of the measured voltage. As measured values, the connected voltages and currents of that measuring
location or side are used to which the voltages are assigned. The fuse failure monitor is, therefore, only
possible for 7UT613 and 7UT633, as 7UT612 and 7UT635 are not equipped with measuring voltage inputs.
The fuse failure monitor can only be used for three-phase protected objects.
If there is substantial voltage asymmetry of the measured values without asymmetry of the currents being
registered at the same time, this indicates the presence of an asymmetrical failure in the voltage transformer
secondary circuit.
The asymmetry of the voltage is detected by the fact that the negative sequence voltage exceeds a settable
value FFM U>(min). The current is assumed to be sufficiently symmetrical if both the zero sequence and the
negative sequence current are below the settable threshold. In at least one phase, the current has to flow
above the limit, as the asymmetry detection cannot function without a minimal measured quantity.
As soon as this state is recognized, all functions that operate on the basis of undervoltage are blocked. The
immediate blocking demands that current flows in at least one of the phases.
If a zero sequence or negative sequence current is detected within approximately 10 s after recognition of this
criterion, the protection assumes a short-circuit in the system 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 (latching of the voltage criterion after 10 s). Only 10 s
after the voltage criterion has been removed by correction of the secondary circuit failure, the blocking will
automatically reset thereby releasing the blocked protection functions again.
Release of the current criterion for
uring locations.
Figure 2-123 shows the assignment to measuring location 1 or side 1 with one assigned measuring location.
That means, for instance, for assignment to side 2, with measuring location 2 and measuring location 3 being
assinged to side 2, the evaluation of addresses 1122 PoleOpenCurr.M2 and 1123 PoleOpenCurr.M3 is
performed.
264
Figure 2-123 depicts the logic diagram of the fuse failure monitor during asymmetrical
VT FuseFail ensues based on the assignment VT SET to selective meas-
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016