I.L. 40-386.3
2-14 (10/94)
when the pilot relay(s) operate and a pilot trip frequency signal from the remote
end is received.
The basic operating concepts of a Simplified Unblocking scheme are the same as the
POTT scheme, except for differences in applied pilot channel equipment. In an un-
blocking scheme; the pilot channel is usually a frequency-shift type power line carrier.
The transmitter frequency must be different at each terminal. It is normally operated
on a blocking frequency and will be shifted to an unblocking frequency when the pilot
relay(s) operate. The carrier receiver should provide logic for which, in the event of
loss-of-channel or low SNR ratio, the pilot trip circuit is automatically locked out after
a short time delay. Pilot trip is provided, however, if the tripping distance relay(s) op-
erate during this short time period between loss-of-channel and pilot trip lockout. ABB
type TCF-10B receiver provides this logic; it provides a 150 ms trip window, then au-
tomatic lockout after loss-of-channel. Provision for a second high-speed pilot trip is
provided, for the situation when a permanent fault causes a permanent loss-of-chan-
nel and the breaker closes onto the fault.
The operating concepts of the pilot distance measurement units Pilot Ø/PLTG are the
same as for the non-pilot Zone distance measurement units, and are supervised by
the same LOP Blk, OSB, IOM, FDOP, and FDOG, units, as shown in Figure 2-20. The
pilot phase and/or pilot ground function(s) can be disabled by setting the Pilot Ø and/
or Pilot G to the OUT position.
The POTT and Simplified Unblocking schemes include the following kinds of logic:
a. Tripping logic (Figure 2-21)
(1) For a forward external fault, the local pilot relay Pilot Ø and/or PLTG sees the fault,
operates and keys. The output from OR-40 will satisfy the first input to AND-30.
Assuming 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 pilot trip should not oc-
cur since the remote transmitter still sends a guard (or blocking) frequency signal.
(2) For an internal fault, the pilot relays at both ends Pilot Ø and/or PLTG see the in-
ternal fault and operate; in addition, the overcurrent supervision output(s),
together with the received trip (or unblocking) frequency signal CR via AND-44
(Figure 2-22), satisfy AND-30 (Figure 2-21). Pilot trip signal PT will be applied to
OR-2 from AND-30. High speed pilot trip (HST) would be obtained. Targets of
PilotØ and/or PLTG will be turned-on after the breaker trips.
b. Carrier Keying Logic (Figure 2-22)
(1) Forward Fault Keying
For a forward internal or external fault, the local pilot relay Pilot Ø and/or PLTG
sees the fault and picks up, operates OR-40, AND-45, OR-18, and AND-35 if pilot
enable is set, and functional display “SystType” is set at POTT position. Output
signal from AND-35 will operate the reed relay (CARSND), key the local transmit-
ter, shift the transmitting frequency from guard to trip (or from a blocking to an
unblocking), to allow the remote pilot relay system to trip.
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