2.28 Measurement Supervision
169
7UM61 Manual
C53000-G1176-C127-3
Thereby I
max
is the largest of the three phase currents I
min
the smallest. The symmetry
factor
BAL. FACTOR I
represents the allowable asymmetry of the phase currents
while the limit value
BALANCE I LIMIT
is the lower limit of the operating range of
this monitoring (see the following figure). Both settings are adjustable. The dropout
ratio is about 95 %.
This malfunction is signaled as
“Fail I balance”
.
Figure 2-64
Current symmetry monitoring
Voltage Symmetry
From the phase-to-ground voltages, the rectified average value is formed as a check
for symmetry of absolute values. The smallest phase voltage is compared to the larg-
est. Asymmetry is recognised if
| U
min
| / | U
max
| <
BAL. FACTOR U
as long as | U
max
| >
BALANCE U-LIMIT
Thereby U
max
is the highest of the three voltages and U
min
the smallest. The symmetry
factor
BAL. FACTOR U
is the measure for the asymmetry of the conductor voltages;
the limit
BALANCE U-LIMIT
is the lower limit of the operating range of this monitoring
(see following figure). Both settings are adjustable. The dropout ratio is about 95 %.
This malfunction is reported as
“Fail U balance”
.
If the 90% stator earth fault protection functions are active, a zero voltage results on
voltage asymmetry. If this cause protection pickup, monitoring is relegated to the back-
ground and issues no indication.
Figure 2-65
Voltage symmetry monitoring
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