Siemens SIPROTEC 4 7VK61 Manual page 135

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Figure 2-52
The asymmetrical measured voltage failure is characterized by its voltage asymmetry with simultaneous
current symmetry. 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 either the zero sequence voltage or the negative
sequence voltage exceed a settable value FFM U>(min) (address 2911). The current is assumed to be suffi-
ciently symmetrical if both the zero sequence as well as the negative sequence current are below the settable
threshold FFM I< (max) (address 2912).
In non-earthed systems (address 207 SystemStarpoint), the zero-sequence system quantities are no reli-
able criterion since a considerable zero sequence voltage occurs also in case of a simple earth fault where a
significant zero sequence current does not necessarily flow. Therefore, the zero sequence voltage is not evalu-
ated in these systems but only the negative sequence voltage and the ratio between negative sequence and
positive sequence voltage.
The immediate effect of the "Fuse failure monitor" is signaled by means of the indication
(No. 170). To detect an asymmetrical measuring voltage failure, at least one phase current must exceed the
value FFM I< (max) (address 2912).
In case that zero sequence or negative sequence current arise within 10 s after detecting an asymmetrical
measuring voltage failure, a short-circuit in the network is assumed and the signal
ately reset. If the zero-sequence voltage or the negative-sequence voltage exceed the presettable value FFM
U>(min) (address 2911) for more than 10 s, the signal
this status, a reset of the signal
sequence current or the negative-sequence current, but only through the fact that the voltages in the zero-
sequence system and in the negative-sequence system fall below the threshold value. The signal
Fail can also be generated independently from the quantity of the phase currents.
During a single-pole automatic reclose dead time, the "Fuse failure monitor" does not detect an asymmetrical
measuring voltage failure. Due to the de-energization in one phase, an operational asymmetry is caused on
SIPROTEC 4, 7VK61, Manual
C53000-G1176-C159-5, Edition 05.2018
Fuse failure monitoring Part 1: Detection of asymmetrical measuring voltage failure
VT FuseFail will no longer be effected by means of an increase of the zero-
VT FuseFail is immedi-
VT FuseFail>10s (No. 169) will be generated. In
Functions
2.7 Monitoring Functions
VT FuseFail
VT Fuse-
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