SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 55 of 74
After the 10ms time delay, a signal is sent, provided that at least one of the voltage
level detectors has reset, which confirms a fault condition.
Since a fast time of 10ms is not desirable for tripping of the local end, a time delay
of 60ms is selected for local tripping and after this delay the trip relay is
energised, after ensuring that at least one of the voltage level detectors has reset.
Aided Trip LED indication is also energised.
For a fault near the remote end (strong infeed), the relay at that end may reset
within 60ms (for a fast breaker operation and relay reset) and signal receive at the
weak infeed terminal may reset before the 60ms time delay of the weak infeed trip
is elapsed and the weak infeed trip may not be achieved. To ensure tripping at the
weak infeed end, a time delay on reset of 100ms is provided on “Signal Receive”.
The weak infeed feature is provided as a standard arrangement with the
permissive overreach scheme, so the weak infeed features, when selected, should
not cause maloperation/malflagging during normal operation of the distance relay
(ie. single phase trip by distance, followed by a three phase trip by “weak infeed”
with “Aided Trip” indication).
If the distance relays operate normally at both ends, the signal receive signal will
still be available even after the relay reset subsequent to fault clearance, due to
delay in the reset of the channel signal. This condition can operate the weak infeed
circuit, causing weak infeed trip and “Signal Send”, unless further action is taken.
To prevent this, a latch circuit is provided to inhibit the weak infeed feature, if any
of the Zone 1/Zone 2 distance elements operate and trip the breaker. Before the
channel signal is received from the other end, the comparators may reset after a
fault clearance, and hence a time delay on drop off of 100ms is provided in the
Zone1/Zone 2 inhibit circuit. The latch resets only after the channel signal resets.
When high source impedances are present, there is the possibility of inadvertent
operation of the weak feed circuit for external faults due to the reverse looking
Zone 3 elements failing to operate to generate the required inhibit signal.
To safeguard against this problem, high set current level detectors of the same
phase as any Zone 2 element must have also operated before the channel signal is
sent to the other end.
A high set current level detector is included in the “signal send” circuit of the
permissive overreach scheme only when “weak infeed” is selected by the switch
SW8, (see Figure 69). Since this feature is required at the strong infeed terminal,
the weak infeed feature has to be selected at both ends when required.
When a single/three phase auto reclose is provided at the strong infeed terminal,
tripping at the weak infeed end may not be desirable, since this provides only 3
phase trips. SW7 is therefore provided to disable the weak infeed trip, but still
provide the weak infeed echo facility to maintain fast clearances at the strong
infeed end.
5.16.27 Blocking scheme with reverse looking Zone 3 elements (BLOCK)
The directional Zone 2 units are required to trip the circuit breakers, provided a
blocking signal (opto isolator input CRX) has not been received from the remote
end of the feeder, indicating that the fault is beyond the remote busbars.
The reverse looking Zone 3 elements are required to send the blocking signal
(85X–1 and 85X–2) for all external faults. However, since the Zone 3 relay will
normally have a certain degree of offset, a blocking signal could be sent for some
internal faults. The directional forward-looking Zone 2 relays are thus used to
cancel the blocking signal for these circumstances.
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