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Measuring Principle
The measuring voltage failure detection is based on the fact a significant negative-
for 1–Pole and 2–
phase sequence system is formed in the voltage during a 1- or 2-pole voltage failure,
Pole Fuse Failures
without influencing the current. This enables a clear distinction from asymmetries im-
pressed 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 - with a too small positive-sequence system - unwanted operation due
to inaccuracies of the measuring voltages failure detection, the function is blocked
below a minimum threshold of the positive-sequence systems of (U
current (I
3–pole Fuse Fault
A 3–pole fuse fault of the voltage transformer cannot be detected by the positive and
negative sequence system as previously described. Here monitoring of the chronolog-
ical sequence of current and voltage is required. If a voltage dip of approximately zero
occurs (or if the voltage is zero), although the current remains unchanged by 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 func-
tion is already present.
Additional Criteria
In addition to this, the function can either be blocked via a binary input or deactivated
by an undervoltage protection at a separate voltage transformer set. If an undervolt-
age is also detected at a separate transformer set, this is most probably not due to a
7UM61 Manual
C53000-G1176-C127-3
< 0,1 I
).
1
N
2.28 Measurement Supervision
< 10 V) and
1
171
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