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I.L. 40-385.5
(c)
Carrier receiving logic (Figure 30
Carrier signal from the receiver output will be
directly applied to AND-47 to disable the pilot
tripping function.
(d)
Channel indication (not shown in
Figure 30)
Since the carrier channel turns on for external
faults only, the channel indicators, SEND and
RCVR, are not memorized.
(e)
Channel simulation (not shown in
Figure 30)
The test function provides the capability to sim-
ulate the TK switch function for keying action
via OR-18 and AND-35 without the operation of
the pilot relay units, and to simulate the RS
switch function for receiving of a blocking signal
without the operation from the remote transmit-
ter.
(f)
Programmable Reclosing Initiation
(Figure 24)
Same as for POTT scheme.
10.2 WEAKFEED TRIP APPLICATION
10.2.1
The logic for a weakfeed terminal is not
required for the BLK system. The BLK system
requires no permissive trip signal from the remote
end, even though the remote end is a weakfeed ter-
minal. The strong end has no problem for tripping an
internal fault. The weak end is usually assumed
either as a "no feed" source, for which it does not
need to trip on an internal fault, or that it can pilot trip
sequentially.
However, the enhanced TBM logic, as described in
above paragraph (10.1.3(b)(2)), may overblock an
internal fault from tripping on weakfeed condition.
The WFEN should be set to YES in order to disable
this enhanced logic.
10.2.2
The logic for a weakfeed terminal is not
required for the PUTT system, because the PUTT
system uses an underreaching relay(s) only for pilot
trip keying, it is impossible to apply this scheme to
protect a system which may have weakfeed condi-
tion.
10.2.3
For POTT and unblocking schemes on the
weak source terminal
10-6
)
(a)
(b)
The basic operating principle of the weakfeed trip
logic for the POTT and simplified unblocking scheme
is as follows:
(A)
On internal faults, the strong source end sends the
trip (or unblocking) frequency signal to the weak end,
and its pilot trip relay(s) will trip, once it receives an
echo trip permission from the weak end. The pilot trip
relay(s) at the weak end cannot pick up due to not
enough internal fault energy, and does not perform
the normal keying function.
(B)
Output of AND-65 (start echo keying), together with
no output from OR-40 (pilot trip relays) and with out-
put from OR-44 (low voltage condition) will satisfy
AND-66. Weakfeed trip will be performed after 50 ms
via OR-2. The timer delay is for coordination because
the voltage trip units are non-directional.
(C)
The single-pole-trip function includes the following
The Z3P/Z3G distance relays should be set for
reverse looking, and
the undervoltage units, VAL, VBL, VCL should
be used.
Echo key for trip permission (Figure 33)
(3)
With one weakfeed condition, when the
weak end receives trip or unblocking sig-
nal, the output from the receiver oper-
ates the echo key logic AND-65,
providing both the pilot relay (from OR-
40) and the reverse looking relay (from
OR-41A) do not pick up, and if system
disturbance is detected (VV or VI). Out-
put of AND-65 will key the weak terminal
transmitter to the trip or unblocking fre-
quency via OR-18 and AND-35.
(4)
On a weak end reverse external fault, the
strong source end sends the trip (or
unblocking) frequency signal to the weak
end, and its pilot trip relay(s) is waiting
for the receiving of the echo trip permis-
sion from the weak end. However, at the
weak end the echo key logic AND-65 will
not operate, because the reverse looking
relay operation sends no echo signal to
the strong end. Both the strong/weak
ends will not trip on this external fault.
Weak end trip on internal fault (Figure 33)
Single-pole trip, SPT (Figure 38)
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