Siemens siprotec 7SA6, Manual

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Functions
6-277
7SA6 Manual
C53000-G1176-C156-2
Figure 6-140 Current symmetry monitoring
Broken Conductor A broken conductor of the protected line or in the current transformer secondary circuit
can be detected, if the minimum current
BALANCE I LIMIT
flows via the feeder. If a
current symmetry failure is detected and the minimum current is below the threshold
PoleOpenCurrent
(address 1130, refer to subsection 6.1.3), an interruption of this
conductor may be assumed. After approximately 5 s the device issues the alarm
“
Fail Conductor
”.
Voltage Symmetry During normal system operation, a certain degree of voltage symmetry can be as-
sumed. The symmetry is monitored in the device with a magnitude comparison. The
smallest phase voltage is compared to the largest. Non-symmetry is detected when
|U
min
| / |U
max
| <
BAL. FACTOR U
as long as |U
max
| >
BALANCE U-LIMIT
U
max
is the largest and U
min
is the smallest of the three voltages. The symmetry factor
BAL. FACTOR U
provides a measure of the voltage unsymmetry, the threshold value
BALANCE U-LIMIT
defines the lower limit of the operating range for this monitoring
function (refer to Figure 6-141). Both parameters can be set. The reset ratio is approx.
97 %.
This failure is alarmed by “
Fail U balance
”.
Figure 6-141 Voltage symmetry monitoring
I
min
I
N
slope:
I
max
I
N
BALANCE I LIMIT
BAL. FACTOR I
U
min
V
BALANCE U-LIMIT
slope:
BAL. FACTOR U
U
max
V