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2.8.1.4 Monitoring of the Transformer Circuits
Open circuits or short circuits in the secondary circuits of the current and voltage transformers, as well as faults
in the connections (important during commissioning!), are detected and reported by the device. The measured
quantities are periodically checked in the background for this purpose, as long as no system fault is present.
Current Symmetry
During normal system operation, symmetry among the input currents is expected. The monitoring of the mea-
sured values in the device checks this balance. The smallest phase current is compared to the largest phase
current. Asymmetry is detected if | Imin | / | Imax | < BAL. FACTOR I as long as Imax > BALANCE I LIMIT
is valid.
Thereby Imax is the largest of the three phase currents and Imin the smallest. The symmetry factor BAL.
FACTOR I (address 8105) represents the allowable asymmetry of the phase currents while the limit value
(address 8104) is the lower limit of the operating range of this monitoring (see Figure 2-34). Both parameters
can be set. The dropout ratio is about 97%.
This failure is therefore located below the curve for all values and is reported as „Fail I balance".
Figure 2-34
Current symmetry monitoring
Voltage Symmetry
During normal system operation, balance among the voltages is expected. Since the phase-to-phase voltages
are insensitive to ground faults, the phase-to-phase voltages are used for balance monitoring. If the device is
connected to the phase-to-ground voltages, then the phase-to-phase voltages are calculated accordingly,
whereas, if the device is connected to phase-to-phase voltages and the displacement voltage V
phase-to-phase voltage is calculated accordingly. From the phase-to-phase voltages, the device generates the
rectified average values and checks the balance of their absolute values. The smallest phase voltage is com-
pared with the largest phase voltage.
Asymmetry is recognized if
| < BAL. FACTOR V as long as | V
| V
| / | V
min
max
three voltages and V
allowable asymmetry of the conductor voltages while the limit value BALANCE V-LIMIT (address 8102) is the
lower limit of the operating range of this monitoring (see Figure 2-35). Both parameters can be set. The dropout
ratio is about 97%.
SIPROTEC, 7SC80, Manual
E50417-G1140-C486-A1, Release date 12.2011
the smallest. The symmetry factor BAL. FACTOR V (address 8103) represents the
min
| > BALANCE V-LIMIT. Where V
max
Functions
2.8 Monitoring Functions
, then the third
0
is the highest of the
max
123
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