ABB REX 521 Technical Reference Manual, General page 21

1MRS 751108-MUM
Protection Relay
Technical Reference Manual, General
The microcontroller and the program execution are supervised by a watchdog once
every 100 ms.
The selector, the A/D converter and other measuring input electronics are tested by
checking a very accurate reference voltage once a minute. This is to ensure that a
measured signal is real and not caused by a fault or disturbance in some input circuit,
all to avoid false output signals.
Setting values are tested with the help of a checksum.
Furthermore, critical setting values are always checked to ensure that the values used
are within the maximum and minimum limits.
The internal supply voltages from the power supply module are tested once a minute
by measuring the voltages, +24 V, +15 V and -15 V.
The trip output paths, the output amplifiers and the output relay coils are checked
once a minute by injecting a 50
current flows through the output relay coils. Both short-circuits and open coils are
detected, since the rise time of the voltage pulse is measured.
Table 4.1.11-1 Self-supervision functions
Supervised object
RAM memories
Nonvolatile memories
Microcontroller and program
execution
A/D converter, multiplexer and
amplifiers
Setting values
HMI (display)
Power supply module
Output amplifiers and relay
coils
When the self-supervision detects a fault, different measures are taken depending on
the severity of the fault. If the fault is fatal, the microcontroller tries to get the system
to work by warm-resetting ten times. If this attempt is not successful, a signal about
the internal relay fault (IRF) is linked to the output relay. Provided that the unit is
operating normally, the information about the fault is sent as an event
"IRF activated" (E31) over the serial communication and the green READY LED
on the front panel starts blinking. An indication about the character of the fault is
also shown as a textual message on the display.
If the fault is considered too dangerous to maintain normal protection, an inoperative
IRF state is entered and no output relay operations are allowed. On the other hand,
if e.g. one output relay is found faulty and the others healthy, normal operations
targeted to a healthy relay are allowed.
µ
s voltage pulse into the circuit and checking that
Supervision method
Write and read of all memory
locations
Checksum
Internal watchdog
Reference voltage
Checksum, correct values
Visual inspection
Measurement of supply
voltages
Feedback from the relay coils 1 min
REX 521
Execution frequency
40 B / 200 ms
When data is fetched
100 ms
1 min
1 min
On connection of supply
voltage
1 min
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