Monitoring Of External Current Transformer Circuits; Xqfwlrqv - Siemens SIPROTEC 7SJ62 Instruction Manual

Functions

6.17.1.3 Monitoring of External Current Transformer Circuits

Current Symmetry
Voltage Symmetry
6-150
Interruptions or short circuits in the secondary circuits of the current transformers or
voltage transformers, as well as faults in the connections (important for start-up!), 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.
During normal system operation (i.e. the absence of a short-circuit fault), symmetry
among the input currents is expected. This symmetry is checked by the device, using
a quantity monitor. The smallest phase current is compared to the largest phase cur-
rent, and asymmetry is recognized if:
| < %$/ )$&725 ,, as long as I
|I |/|I
min max
where I is the largest of the three phase currents and I
max
metry factor %$/ )$&725 , represents the allowable asymmetry of the phase cur-
rents while the limit value %$/$1&( , /,0,7 is the lower limit of the operating range
of this monitoring (see Figure 6-62). Both settings are adjustable, and the dropout ratio
is about 97 %.
This malfunction is reported as "0DOIXQFWLRQ ,V\PP".
I
min
I
N
76G6I8@ÃDÃGDHDU
Figure 6-62 current Symmetry Monitoring
During normal system operation (i.e. the absence of a short-circuit fault), symmetry
among the input voltages is expected. Because the phase-to-phase voltages are in-
sensitive to ground connections, the phase-to-phase voltages are used for the sym-
metry monitoring. If the device is connected to the phase-to-ground voltages, then the
phase-to-phase voltages are calculated accordingly, whereas if the device is connect-
ed to phase-to-phase voltages and the displacement voltage, then the third phase-to-
phase voltage is calculated accordingly. From the phase-to-phase voltages, the small-
est and largest phase-to-ground voltages are calculated and compared to check for
symmetry. Asymmetry is recognized if:
| < %$/)$&725 9, as long as |V
|V |/|V
min max
where V is the largest of the three phase-to-ground voltages and V
max
est. The symmetry factor %$/)$&725 9 is the measure for the asymmetry of the
conductor voltages; the limit value %$/$1&( 9²/,0,7 is the lower limit of the oper-
ating range of this monitoring (see Figure 6-63). Both settings are adjustable. The
dropout ratio is about 97 %.
This malfunction is reported as "0DOIXQFWLRQ 9V\PP".
> %$/$1&( , /,0,7 / I
/I
max N
is the smallest. The sym-
min
Increase:
76GA68UPSÃD
I
max
I
N
| > %$/$1&( 9²/,0,7
max
N
is the small-
min
7SJ62 Manual
C53000-G1140-C121-1