Functions
2.24 Monitoring Functions
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
420
Measured value acquisition voltages
Four measuring inputs are available in the voltage path: three for phase-to-earth voltages and one input for the
displacement voltage (e-n voltage of open delta winding) or a busbar voltage. If the displacement voltage is
connected to the device, the sum of the three digitized phase voltages must equal three times the zero se-
quence voltage. Errors in the voltage transformer circuits are detected when
U
F
= |U
L1
+ U
L2
+ U
L3
+ k
U
·U
EN
| > 25 V.
The factor k
U
allows for a difference of the transformation ratio between the displacement voltage input and the
phase voltage inputs (address
211
Uph / Udelta
).
This malfunction is signalled as
„Fail
Σ
U Ph-E“
(no. 165).
Note
Voltage sum monitoring is only effective if an external displacement voltage is connected to the displacement
voltage measuring input.
Voltage sum monitoring can operate properly only if the adaptation factor
Uph / Udelta
at address
211
has
been correctly configured (see Subsection 2.1.2.1).
2.24.1.2 Software Monitoring
Watchdog
For continuous monitoring of the program sequences, a time monitor is provided in the hardware (watchdog for
hardware) that expires upon failure of the processor or an internal program, and causes a reset of the processor
system with complete restart.
An additional software watchdog ensures that malfunctions during the processing of programs are discovered.
This also initiates a restart of the processor system.
If the fault is not eliminated by the restart, a second restart attempt is initiated. If the fault is still present after
three restart attempts within 30 s, the protection system will take itself out of service, and the red LED „ERROR“
lights up. The device ready relay drops out and alarms the device malfunction with its normally closed contact
(„life contact“).
2.24.1.3 Measurement Circuit Monitoring
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. To this end, the
measured values are cyclically checked in the background as long as no fault detection is present.
Current Symmetry
During normal system operation the currents are assumed to be largely symmetrical. The symmetry is moni-
tored in the device by magnitude comparison. The smallest phase current is compared to the largest phase
current. Asymmetry is recognized if:
|
I
min
| / |
I
max
| <
BAL. FACTOR I
as long as
I
max
>
BALANCE I LIMIT
I
max
is the highest,
I
min
the lowest of the three phase currents. The symmetry factor
BAL. FACTOR I
(address
2905
) represents the allowable asymmetry of the phase currents while the limit value
BALANCE I
LIMIT
(address
2904
) is the lower limit of the operating range of this monitoring (see Figure 2-203). The
dropout ratio is about 97%.
