SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 49 of 74
5.16.7 Output seal-in
If a failure of the voltage supply occurs, the VTS instantaneously blocks the distance
relay from tripping. It is necessary to seal in the blocking action, since otherwise if
a ground fault were to occur on the transmission line later, the
I
0
would reset the
VTS. A 5.5s delay is provided before sealing in the blocking action, so that the
VTS can tolerate temporary voltage failures without permanently blocking the
distance relay. An example of a temporary failure is a momentary short circuit on
the voltage supply. A failure which lasts for over 5.5s is deemed to be permanent
and in the case of a short circuit, usually results in a fuse blocking. The visual
indication and electrical alarms are only given after the 5.5s has elapsed, so that
an alarm is only given for permanent failures.
5.16.8 Operation for indication and alarm only
If switch SW3 is set to the left hand position, so that the VTS can only indicate and
give an alarm, a failure of the voltage supply may cause this distance relay to trip
immediately. The ac supply would then be removed and the V
0
level detector reset
before the 5.5s had elapsed.
A means of accelerating the indication and alarm is therefore provided which only
operates if a comparator operates when the VTS detects a voltage supply failure.
This is shown on the block diagram. It can be seen that there is a 20ms delay
before the accelerated indication is allowed. this prevents any possibility of a
transient pulse from the detection circuits causing incorrect alarms.
Such pulses (typically up to 2–3ms) can sometimes occur for transmission line faults
under conditions where the
I
0
level detector is slowed and also when faults are
cleared by opening the circuit breaker, when there is a race between resetting of
the
I
0
and V
0
and low set current level detectors.
5.16.9 Resetting
The equivalent of bistable circuits are used to seal in the block and indication/
alarm outputs of the VTS. It is necessary to visit the relay room and press the RESET
pushbutton on the relay to reset the bistables. This can conveniently be done at the
same time as repairing the voltage supply failure and/or replacing the faulty fuse.
If the RESET pushbutton is pressed while the voltage supply is still faulty and the
line energised, the comparators remain blocked and the indication/alarm output
will be retained.
5.16.10 Line de-energisation (line VTs)
After all three poles of the circuit breaker have been opened, there may be
sufficient unbalance in any induced voltage from an adjacent line to cause the V
0
detector to operate. The line may remain in a de-energised state for longer than
5.5s and so a circuit is provided to prevent the block and indication/alarm outputs
from sealing in.
The circuit is shown in the block diagram and uses the “ALL POLES DEAD” signal,
derived from the current and voltage level detectors or “BREAKER OPEN” opto to
block the input to the 5.5s timer.
If the pole scatter on opening of the breakers is sufficiently spread, transient
operation of the V
0
detector may occur. To prevent this bringing up an indication
via the logic, a latch is provided which ensures that the indication can only pass to
the output latch provided that the V
0
detector operated before the comparators
picked up.
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