Siemens SIPROTEC 7SD610 Manual page 207

uated in these systems but only the negative sequence voltage and the ratio between negative sequence and
positive sequence voltage.
As soon as such an error has been detected, all functions that work on the basis of undervoltage are blocked.
The immediate blocking requires that current flows in at least one of the phases. The differential protection can
be switched to emergency operation, provided that these functions are parameterized accordingly (refer also
to Section 2.9).
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).
Figure 2-89 Fuse Failure Monitor Part 2: Detection of the 3-phase measuring voltage failure
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011
Functions
2.15 Monitoring Functions
207