SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 50 of 74
5.16.11 Busbar voltage transformers
When the relay is supplied from a busbar VT there is no loss of ac voltage when
the circuit breaker is opened. However, there may be a transient depression of the
voltage on reclosing the breaker, caused by line charging current, magnetising
inrush, etc. Energisation of the “BREAKER OPEN” opto isolator is therefore
arranged to inhibit the instantaneous output of the VTS (via the “ANY POLE DEAD”
logic).
5.16.12 Single-pole tripping
An additional feature is provided for power systems with single pole tripping of
single phase to ground faults. When a single pole of a transmission line is de-
energised, the unbalance in the voltage is likely to cause the V
0
detector to
operate. The single pole dead condition will not last for as long as 5.5s, so there is
no need to stop the time delayed output from the VTS. However, the instantaneous
output must be stopped, otherwise the relay comparators would not be able to trip
if a phase-phase fault were to occur on the remaining live phases, since a phase-
phase fault does not cause any
I
0
to reset the VTS.
The instantaneous output is stopped by an “ANY POLE DEAD” signal obtained
from the three “POLE DEAD” signals (or BREAKER OPEN OPTO if connected).
A 240ms delay on drop off of this “ANY POLE DEAD” inhibition is provided
because line charging transients and CVT transients may cause the V
0
detector to
remain operated for a short time after breaker reclosure.
A side effect is that if a single phase of the voltage supply fails at a time when
there is insufficient line current to operate the low set current detector of that phase,
an “ANY POLE DEAD” signal is produced. The instantaneous blocking output is
then inhibited, though the time delayed sealed-in output is still produced after 5.5s.
This is not a practical disadvantage, since no comparator maloperation can occur
if the low set phase current detector has not operated.
5.16.13 Switch on to fault
If the transmission line is energised with one or two VT fuses missing, the
subsequent action depends on the load current flowing in the line. If the load
current is high enough to pick up one or more low set phase current detectors
within the first 240ms of the line being live, a switch-on-to-fault trip results.
However, if none of the low set phase current detectors operates in this time, the
VTS blocks the relay from tripping and seals-in and gives alarm and indication
after 5.5s.
If the transmission line is energised with all three fuses missing, the switch-on-to-fault
feature remains enabled until the load current is high enough for one or more low
set current detectors to operate, whereupon a switch on to fault trip is produced.
When the relay is used with miniature circuit breakers the switch-on-to-fault circuit is
blocked when the mbc is open.
5.16.14 Weak infeed (POR scheme option) during fuse failure conditions
For a weak infeed fault condition, the VTS can also operate and hence the
instantaneous output cannot be used to inhibit weak infeed logic. However, for true
fuse failure conditions the output from the time delayed (5.5s) signal can be used
for inhibiting weak infeed operation.
The opening of the miniature circuit breaker will block operation of the weak
infeed circuit.
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