Siemens SIPROTEC 7SD610 Manual page 206

Functions
2.15 Monitoring Functions
Fast Asymmetrical Measuring Voltage Failure "Fuse Failure Monitor".
In the event of a measured voltage failure due to a short circuit fault or a broken conductor in the voltage trans-
former secondary circuit certain measuring loops may mistakenly see a voltage of zero. Simultaneously existing
load currents may then cause a spurious pickup.
If fuses are used instead of a voltage transformer miniature circuit breaker (VT mcb) with connected auxiliary
contacts, then the „Fuse Failure Monitor" can detect problems in the voltage transformer secondary circuit. Of
course, the VT miniature circuit breaker and the „Fuse Failure Monitor" can be used at the same time.
Figures 2-88 and 2-89 depict the logic diagram of the „Fuse Failure Monitor".
Figure 2-88 Fuse failure monitoring Part 1: Detection of asymmetrical measuring voltage failure
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-
206
SIPROTEC, 7SD610, Manual
C53000-G1176-C145-6, Release date 02.2011