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or unblocking by the transmitter during the fault. The
memorized RCVR indicator will be displayed after
the breaker trips and a carrier trip signal is received
from the receiver.
e. TBM, Transient Block and Unblock Logic
For a loop system or a paralleled line application,
power reversal may introduce problems to the pilot
relay system especially when a 3-terminal line is in-
volved, since the distance units may have to be set
greater than 150% of ZL in order to accommodate
the infeed effect from the tapped terminal. They may
see the external fault on the parallel line when the
third source is out of service. The transient block and
unblock logic (TBM) is used to solve this problem.
There are some other typical cases of the protected
line being tripped by a ground directional relay upon
clearing of a fault in the adjacent (but not parallel)
line. When the adjacent line breaker trips, it inter-
rupts the current in the faulted phase as well as the
load current in the unfaulted phases. Dependent on
the direction of this load current, and the contact
asymmetry of the breaker, there can be a short pulse
of load-derived I
with possible "tripping direction"
o
polarity, which provides an electrical forward-torque
to the ground directional relay. Therefore, it is de-
sired to increase the security and the transient block
timer (0/50) logic will be included automatically in the
application if STYP=POTT is set. For POTT appli-
cation, the Z3FR setting should be set to "REV"
and Z3P, Z3G should be set to 100% of the line
impedance.
f. Channel Simulation
The test function selection provides the capability to
simulate the TK switch function for keying action via
OR-18 and AND-35 without the operation of pilot re-
lay units, and to simulate the RS switch function for
receiving of a trip or unblocking frequency signal ac-
tion without the operation from the remote transmit-
ter.
g. Programmable Reclosing Initiation (Figure 3-19)
The basic programmable RI application is as de-
scribed in Section 3.4.14. However, on pilot sys-
tems, to activate the RI2 on any 3-pole high-speed
trip, the external pilot enables switch should be ON,
and the PLT and Z1RI should be set to YES. The op-
(5/92)
eration will occur via the logic AND-89, AND-84 and
OR-84A (as shown in Figure 3-19).
3.5.2 Permissive Underreach Transfer Trip
(PUTT)
The basic operating concepts of a PUTT scheme
are:
(1) Pilot relays (PLTP/PLTG) are set to over-
reach. The pilot channel is a frequency-shift
type device, and the transmitter frequency
should be different at each terminal; its signal
may be passed through metallic wire or
microwave.
(2) Channel is normally operated with a guard
frequency, the channel frequency will be
shifted from guard to trip when the Zone 1
reach relay (Z1P/Z1G) operates, and pilot
trip is performed when the pilot relay (PLTP
and/or PLTG) operates, together with the re-
ceiving of a carrier trip signal from the remote
end.
PUTT includes the following logic: the functional dis-
play (STYP) should be set to PUTT position.
a. Pilot Tripping Logic
The Pilot Tripping Logic for the PUTT scheme is ex-
actly the same as for the POTT scheme (Figures 3-
21, 3-22).
b. Carrier Keying Logic
(1) Forward fault keying (Figure 3-24)
For a forward end zone fault, the PUTT
scheme will not key except when the internal
fault is within Zone 1. This means that the
PUTT scheme keys only on Zone 1 faults.
Keying flows via AND-46, OR-18 and AND-
35.
(2) Signal continuation (Figure 3-23)
Same as for POTT scheme.
The TBM logic is not required because the carrier
keying units are set underreach.
NOTE: For open breaker condition, the echo
keying will not work due to lack of the
"SEND" signal from the remote terminal
for an end zone fault. The remote termi-
nal relies on Zone 2 to clear the fault.
I.L. 40-385.1B
3-11
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