Siemens SIPROTEC 7SA6 Manual page 467

Broken Conductor
Voltage Symmetry
7SA6 Manual
C53000-G1176-C156-2
I
min
I
N
BALANCE I LIMIT
Figure 6-140 Current symmetry monitoring
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 ".
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
| < BAL. FACTOR U
|U | / |U
min max
| > BALANCE U-LIMIT
as long as |U
max
U is the largest and U
max
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 ".
U
min
V
BALANCE U-LIMIT
Figure 6-141 Voltage symmetry monitoring
slope:
BAL. FACTOR I
I
max
I
N
is the smallest of the three voltages. The symmetry factor
min
slope:
BAL. FACTOR U
U
max
V
Functions
6-277