Monitoring Of The Transformer Circuits - Siemens SIPROTEC 7SJ62 Manual

2.11.1.5 Monitoring of the Transformer Circuits

Interruptions or short circuits in the secondary circuits of the current and voltage transformers, as well as faults
in the connections (important for commissioning!), are detected and reported by the device. The measured
quantities are cyclically checked in the background for this purpose, as long as no system fault is present.
Current Symmetry
During normal system operation, a certain symmetry among the input currents is expected. The monitoring of
the measured 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 / I
I LIMIT / I
Nom
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
8105 (address 8104) is the lower limit of the operating range of this monitoring (see Figure 2-66). Both param-
eters can be set. The dropout ratio is about 97%.
This malfunction is reported as „Fail I balance".
Figure 2-66
Voltage Symmetry
During normal system operation (i.e. the absence of a fault), balance among the input voltages is expected.
Because the phase-to-phase voltages are insensitive to ground connections, 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, then the third phase-to-phase voltage is calculated accordingly. Whereas if the
device is connected to phase-to-phase voltages and the displacement voltage V
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 compared with
the largest phase voltage. Imbalance is recognized if:
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
.
Current symmetry monitoring
Functions
2.11 Monitoring Functions
> BALANCE
Nom
, then the third phase-to-
0
201