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2.42.2
Supervision
2.42.2.1 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. The measuring results
of the undervoltage protection, the impedance protection and other voltage-dependent protective functions
may be falsified in this way, possibly causing an unwanted operation.
If fuses are used instead of a secondary miniature circuit breaker (VT mcb) with connected 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.
This function uses the current of side 2.
Measuring Principle for 1-Pole and 2-Pole Fuse Failures
The measuring voltage failure detection is based on the fact a significant negative-phase sequence system is
formed in the voltage during a 1- or 2-pole voltage failure, without influencing the current. This enables a clear
distinction from asymmetries impressed by the power system. If the negative-phase sequence system is
related to the current positive-phase sequence system, the following rules apply for the fault-free case:
If a fault of the voltage transformers occurs, the following rules apply for a single-pole failure:
If a fault of the voltage transformers occurs, the following rules apply for a two-pole failure:
In case of an outage of one or two phases, the current also shows a negative-phase sequence system of 0.5
or 1. Consequently, the voltage monitoring does not respond since no voltage transformer fault can be present.
In order to avoid - in case of a too small positive-sequence system - an unwanted operation by inaccuracies of
the measuring voltages failure detection, the function is blocked below a minimum threshold of the positive-
sequence systems of voltage (U
3-pole Fuse Failure
A 3–pole fuse failure of the voltage transformer cannot be detected by the positive and negative sequence
system as previously described. Here monitoring of the chronological sequence of current and voltage is re-
quired. If a voltage dip of approximately zero occurs (or if the voltage is zero), although the current remains
unchanged at the same time, this is probably due to a 3-pole voltage transformer failure. The deviation of the
actual current value from the nominal current value is evaluated for this purpose. The measuring voltage failure
monitoring is blocked if the deviation exceeds a threshold value. Moreover, this function is blocked if a pickup
of an (overcurrent) protective function is already present.
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
< 10 V) and current (I
< 0.1 I
1
1
2.42 Monitoring Functions
).
N
Functions
289
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