Processor; Internal Power Supplies; Analog/Digital Converters; Trip Circuits - Siemens 7UT51 Instruction Manual

12
Protection Self-Monitoring
12.2.6

Processor

For continuous monitoring of the program sequences,
a watchdog timer is provided which will reset the
processor in the event of processor failure or if a
program falls out of step. Further, internal plausibility
checks ensure that any fault in processing of the
programs, caused by interference, will be recognized.
Such faults lead to reset and restart of the processor.
If such a fault is not eliminated by restarting, further
restarts are initiated. If the fault is still present after
three restart attempts the problem is considered
persistent and the relay places itself out of service.
12.2.7

Internal Power Supplies

The relay monitors the internal voltage supplies used
to power internal components of the relay. These
include voltages for the multiplexer and offset and
reference voltages for the ADC (analog/digital
converter). Between the corresponding coming and
going events, the relay is not ready.
Failure or switch-off of the auxiliary voltage
automatically puts the system out of operation. This
status is indicated by a fail-safe contact and
announced in events 141, 142, and 143.
12.2.8

Analog/Digital Converters

If the relay detects a problem of the analog/digital (A/
D) converters, event 145C occurs and the relay is not
ready.
12.2.9

Trip Circuits

The functioning of the trip contacts of the relay are
monitored by internal software.
The trip contacts are controlled by two command
channels and one additional release channel. If
discontinuities or short circuits in the circuits
controlling the trip contacts are detected, the relay is
not ready.
120
12.3 Monitoring of Supply
Power
Failure or switch-off of the DC supply voltage will
de-energize the Self-Monitor signal contact (which is
normally energized, see Section 12.2 on page 119).
No events or other actions occur, and, of course, the
relay is no longer providing any protection functions.
Transient dips in the DC supply voltage for less than
50 ms (for VDC > 110V) will not disturb the functioning
of the relay.
12.4 Monitoring of CT
Circuits
The relay can detect interruptions or short circuits in
the current transformer (CT) circuits by monitoring the
plausibility of the measured current values. The values
are checked at cyclic intervals, so long as no
protection function is in pickup.
Note that implausible measured current values
detected while commissioning the relay may indicate
incorrect connection of the CT's to the relay's
current-input terminals.
The sensitivity of the measured values monitoring can
be changed in Address Block 2900. The factory
settings should be adequate in most cases. When
using the 7UT51 as protection for a single-phase
transformer, both parameters should be set to the
highest value since otherwise an alarm could be given
continuously.
12.4.1

Current Symmetry

During healthy operation, it can be expected that the
three phase currents for each protected winding (or
bus branch) will be approximately symmetrical.
Specifically, if I is the smallest of the three phase
min
currents and I is the largest, the ratio of their
max
magnitudes will be unity when the currents are exactly
symmetrical.
7UT51 v3
PRIM-2330C