Siemens siprotec 7SA6 User Manual page 438

Functions
Broken Conductor
Voltage Symmetry
Voltage Phase
Rotation
6-256
A broken conductor of the protected line or in the current transformer secondary circuit
can be detected, if the minimum current %$/$1&( , /,0,7 flows via the feeder. If a
current symmetry failure is detected and the minimum current is below the threshold
3ROH2SHQ&XUUHQW (address 1130, refer to subsection 6.1.3), an interruption of this
conductor may be assumed. After approximately 5 s the device issues the alarm
")DLO &RQGXFWRU".
During normal system operation, a certain degree of voltage symmetry can be
assumed. The symmetry is monitored in the device with a magnitude comparison. The
smallest phase voltage is compared to the largest. Non-symmetry is detected when
| < %$/ )$&725 8
|U | / |U
min max
| > %$/$1&( 8/,0,7
as long as |U
max
U is the largest and U
max
%$/ )$&725 8 provides a measure of the voltage unsymmetry, the threshold value
%$/$1&( 8/,0,7 defines the lower limit of the operating range for this monitoring
function (refer to Figure 6-132). Both parameters can be set. The reset ratio is approx.
97 %.
This failure is alarmed by ")DLO 8 EDODQFH".
U
min
V
76G6I8@ÃVGDHDU
Figure 6-132 Voltage symmetry monitoring
The verification of the faulted phases and the phase preference, direction
measurement and polarization with quadrature voltages usually demand clockwise
rotation of the measured values. The phase rotation of the measured voltages is
checked by monitoring of the voltage phase sequence.
U before U before U
L1 L2
This check takes place if each measured voltage has a minimum magnitude of
|U |, |U |, |U | > 40 V/√3
L1 L2 L3
In the event of negative phase rotation, the alarm ")DLO 3K 6HT" is issued.
If the system has a negative phase rotation, this must have been set during the
configuration of the power system data (Sub-section 6.1.1). In such event, the phase
rotation monitoring applies to the corresponding opposite phase sequence.
is the smallest of the three voltages. The symmetry factor
min
slope:
76GÃA68UPSÃV
U
max
V
L3
7SA6 Manual
C53000-G1176-C133-1