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Functions
2.24 Monitoring Functions
The asymmetrical measured voltage failure is characterised 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 se-
quence 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 eval-
uated in these systems but only the negative sequence voltage and the ratio between negative sequence and
positive sequence voltage.
As soon as this state is recognized, the distance protection and all other functions that operate on the basis of
undervoltage (e.g. also weak infeed tripping) are blocked. The indication „VT FuseFail" (No. 170) is output.
The immediate blocking requires that current flows in at least one of the phases. The distance protection can
be switched to differential protection and/or O/C emergency operation, provided that these functions are pa-
rameterized accordingly (refer also to Sections 2.3 and 2.16).
The immediate effect of the „Fuse failure monitor" is signaled by means of the indication „VT FuseFail" (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 mea-
suring voltage failure, a short-circuit in the network is assumed and the signal „VT FuseFail" is immediately
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 „VT FuseFail>10s" (No. 169) will be generated.
In this status, a reset of the signal „VT FuseFail" will no longer be effected by means of an increase of the
zero-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 „VT
FuseFail" 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
the primary side which cannot be distinguished from a measuring voltage failure in the secondary circuit (not
represented in the logic diagram).
426
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
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