Siemens SIPROTEC 7UT613 series Manual page 283

Asymmetrical Mea-
suring Voltage
Failure "Fuse
Failure Monitor".
7UT613/63x Manual
C53000-G1176-C160-2
>0.1 · I
), or no zero crossing is registered. The currents flowing in other phases must
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not exceed 2 I
.
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The differential protection and the restricted earth fault protection are blocked imme-
diately in the relevant measuring location. The protection functions which react on un-
symmetrical currents are blocked as well provided they are assigned to the defective
measuring location: the time overcurrent protection for residual current and the unbal-
anced load protection. The device issues the message „Broken wire" also indicating
the affected phase and measuring location.
Blocking is cancelled as soon as the device is again supplied with a current in the rel-
evant phase.
Detection of a broken wire is restricted by technical limits. A broken wire in the sec-
ondary circuit can, of course, only be detected when a steady state current has been
flowing through the respective phase. Furthermore, a wire break at the instant of zero
crossing in current cannot always be detected reliably. No expected value can be
created when the frequency is out of the operation frequency (f
Note that electronic test devices do not simulate the correct behaviour of broken wire
so that pickup may occur during such tests.
In the event of a measured voltage failure due to a short circuit fault or a broken con-
ductor in the secondary circuit certain protection and monitoring functions (whose op-
eration is based on undershooting a measuring voltage) can be spuriously triggered.
This delays an unauthorised trip. For 7UT613/63x this applies to forward power super-
vision P< and the undervoltage protection.
If fuses are used instead of a secondary miniature circuit breaker (VT mcb) with con-
nected auxiliary contacts, then the („fuse failure monitoring") can detect problems in
the voltage transformer secondary circuit. Of course the 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 asymmet-
rical with simultaneous current symmetry. Figure 2-116 depicts the logic diagram of the
„Fuse Failure Monitors" during asymmetrical failure of the measured voltage. As mea-
sured values the connected voltages and currents of the measured locations or sides
are used that are assigned voltages. The fuse failure monitor is therefore only possible
for 7UT613 and 7UT633, as 7UT635 it is 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 the settable value FFM U>(min). The current is assumed to be suf-
ficiently symmetrical if both the zero sequence as well as 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 cannot function without a minimal measured quan-
tity.
As soon as this is recognised all functions that operate on the basis of undervoltage
are blocked. 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 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
2.19 Monitoring Functions
± 10 %).
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