SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 56 of 74
In practice a time delay on pick up for the tripping signal is required for faults
detected by the Zone 2 elements. This is to allow time for a blocking signal to be
sent from the remote end, should it prove necessary, and to be received at the
local end. Such a pick up timer is provided by Quadramho scheme logic and is
user adjustable by the positions of 4 switches (t
p
) selected on the frontplate of the
scheme logic module. A range of 0–90ms in 6ms steps may be set for t
p
.
A delay on drop-off for the “signal received” (CRX) is also provided in the blocking
scheme, to safeguard against inadvertent tripping out of the healthy section of a
faulted double circuit line, during a possible reversing current situation.
This timer is also user adjustable by means of 4 switches (t
d
) on the frontplate of
the scheme logic module, to give a range of 0–90ms in 6ms steps. Figure 71 gives
the equivalent logic diagram of the blocking scheme.
When very high source impedances are present, there is the possibility of
inadvertent tripping occurring for external faults due to the reverse Zone 3
elements failing to operate to generate the required blocking signal. To safeguard
against this problem, high set current level detectors (LDHS) of the same phase as
any Zone 2 element must also have picked up before an accelerated aided trip
action can be initiated.
5.16.28 Zone 1 extension scheme (Z1EXT)
This scheme does not require a signalling channel.
The Zone 1 relays are normally in the Zone 1 extend mode and are reset to the
Zone 1 reach when the circuit breakers have tripped and are about to be reclosed
by auto-reclose action. The signal to reset from the extended Zone 1 to the normal
Zone 1 is generated by the auto-reclose equipment, via the opto isolator input
RZ1X (reset Z1X).
The scheme provides fast clearance of most faults. On the basis that most overhead
line faults are transient in duration, the scheme will allow fast clearance of most
faults along the protected section and also those just out of the section. Lack of
discrimination does not matter as auto-reclosure of the protected section circuit
breaker(s) will take place. The operation of the auto-reclose relay is used to reset
the extension facility so that if the fault is permanent, upon reclosure the faulted
section of line will be cleared permanently by its own protection, as in the basic
scheme. Refer to application notes.
5.16.29 Opto-isolator inputs to scheme logic
Five opto isolators are provided in Quadramho. These allow the scheme logic to
access information from external equipment and take appropriate action, as
described below:
a) Breaker open opto isolator.
This is used primarily where the relay is supplied via busbar voltage
transformers and therefore the internal relay “pole dead” detection circuits are
inoperative (See Figure 64). It is energised from a contact on the circuit
breakers, which is closed when all poles of the breaker are open. The “Breaker
Open” opto is used to:
i) enable the switch-on-to-fault feature (via pole dead logic).
ii) enable the “echo” feature of the permissive overreach scheme.
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