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I.L. 40-385.1B
loss-of-channel and the breaker closes onto the
fault.
Indication for low signal and loss-of-channel condi-
tion will not be provided for this simplified unblocking
scheme. (This function would normally be incorpo-
rated in the power line carrier equipment.)
The operating concepts of the pilot distance mea-
surement units (PLTP/PLTG) are the same as for
the non-pilot zone distance measurement units, and
are supervised by the same LOPB, OSB, IOM,
FDOP, and FDOG, units, as shown in Figure 3-21.
The pilot phase and/or pilot ground function(s) can
be disabled by setting the PLTP and/or PLTG to the
OUT position.
The POTT and Simplified Unblocking schemes in-
clude the following kinds of logic:
a. Tripping logic (Figure 3-22)
(1) For a forward external fault, the local pilot re-
lay (PLTP and/or PLTG) sees the fault,
operates and keys. The output from OR-40
will satisfy the first input to AND-30. Assum-
ing that reverse block (TBM) logic does not
operate and pilot enable is set, then three out
of four inputs of AND-30 are satisfied, but pi-
lot trip should not occur since the remote
transmitter still sends a guard (or blocking)
frequency signal.
(2) For an internal fault, the pilot relays at both
ends (PLTP and/or PLTG) see the internal
fault and operate; in addition, the overcurrent
supervision output(s), together with the re-
ceived trip (or unblocking) frequency signal
CR via AND-44 (in Figure 3-23), satisfy AND-
30 (in Figure 3-22). Pilot trip signal PT will be
applied to OR-2 from AND-30. High speed
pilot trip (HST) would be obtained. Targets of
pilot phase trip (PLTP) and/or pilot ground
trip (PLTG) will be turned-on after the break-
er trips.
b. Carrier Keying Logic (Figure 3-23)
(1) Forward Fault Keying
For a forward internal or external fault, the lo-
cal pilot relay (PLTP and/or PLTG) sees the
fault and picks up, operates OR-40, AND-45,
3-10
OR-18, and AND-35 if pilot enable is set, and
functional display "STYP" is set at POTT po-
sition. Output signal from AND-35 will
operate the reed relay (CARSND), key the
local transmitter, shift the transmitting fre-
quency from guard to trip (or from a blocking
to an unblocking), to allow the remote pilot
relay system to trip.
(2) Echo Keying
Since the POTT and the Simplified Unblock-
ing schemes require the receiving of a
permissive signal from the remote end, for pi-
lot trip, provision should be made for
covering the condition when the remote
breaker is opened.
When the remote breaker is opened, the in-
puts of AND34B (Figure 3-23) will satisfy the
NOT FORWARD, NOT REVERSE and
52b=1 conditions. Any carrier RCVR (CR)
will produce an output from AND 34B and
also a SEND signal from AND 35 via OR-18.
This echo keying will be stopped by itself af-
ter 150 ms due to the input timer of AND 34B.
(3) Signal Continuation
This logic includes the signal of TRSL, 0/150
ms timer, OR-18, and AND-35. The 0/150 ms
signal continuation time is required to keep
the local transmitter at the trip frequency (or
unblocking) for 150 ms after the local end
high speed trips which includes pilot trip,
Zone 1 trip, and high-set overcurrent trip, in
case of sequential trip on the system. This
logic will be disabled by the signal via TDT,
the 0 / 300 ms timer (AND-34A) on any time
delay trip operation, and will also be dis-
abled by CIF trip (AND 49A).
c. Carrier Receiving Logic (Figure 3-23)
This logic includes OR-15 (not shown) and AND-44.
Output "trip" (or unblocking) frequency signal from
the carrier receiver operates the logic OR-15 and will
produce a carrier trip (CR) signal from AND-44.
d. Channel Indicators (Figure 3-23)
The memorized SEND indicator will be displayed af-
ter the breaker trips and the frequency shifts to trip
(5/92)
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