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3.17 UNEQUAL-POLE CLOSING LOAD PICKUP
LOGIC (See Section 3.4.8)
3.18 SELECTABLE LOSS-OF-LOAD
ACCELERATED TRIP LOGIC (LLT) (See
Section 3.4.9)
3.19 CURRENT CHANGE FAULT DETECTOR
(See Section 3.4.10)
3.20 VOLTAGE CHANGE FAULT DETECTOR
(See Section 3.4.10)
3.21 3-TERMINAL LINE APPLICATION
For a 3-terminal BLK application, since the frequen-
cy of the three transmitters are the same, any one
transmitter starting will block the pilot system from
tripping, therefore, logic for the 3-terminal BLK pilot
system would be the same as that used for the 2-ter-
minal BLK system. However, for POTT/PUTT/UBLK
systems, since the transmitter frequency is different
at each terminal, logic for the second receiver
(RCVR-2) should be added to the system when the
application involves 3-terminals. Functional display
"3TRM" should be set at "YES" position when the 3-
terminal line is applied.
a. Additional Logic For POTT and Simplified Un-
blocking (Figure 3-29)
This logic includes contact converters (CC) for
RCVR-2, AND-55, and logic for the second receive
indication. The second receiver output operates the
contact converter (or voltage). Output of AND-55
provides carrier trip signal (CR) to satisfy AND-30 via
AND-64 and allows pilot tripping.
b. Additional Logic for PUTT (Figure 3-30)
The additional logic for this scheme would be similar
to that as described for POTT scheme, except logic
includes AND-56, AND-57 and 50/ms timer. This is
because only the Zone 1 reach relay keys the trans-
mitter on internal faults. For a close-in Zone 1 fault,
only the local terminal can key its transmitter and the
other two cannot. This logic provides a CR pilot trip
signal for 50 ms for system security. For a fault which
can be detected by relays at two terminals, AND-55
logic can be satisfied, then pilot trip will be performed
via the logic in the usual way.
(5/92)
3.22 WEAKFEED TRIP APPLICATION
a. Block/Weakfeed
The logic for a weakfeed terminal is not required for
the BLK system because the BLK system requires
no permissive trip signal from the remote end, even
though the remote end is a weakfeed terminal. The
strong end has no problem tripping for 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 it can pilot trip sequentially.
NOTE: Refer to Figure 3-25, logic AND-41B and
OR-41C, WFEN should be set to YES if
OSB is set to YES and this terminal may
become a weak condition.
For the bench test, at the conditions of V
= 0 and I = 0, the carrier keying contacts
will be closed for the settings of OSB =
YES and WFEN = NO. In an actual sys-
tem, 52b will be applied to OR41C, be-
cause of V = 0 and I = 0, and the carrier
keying signal will not be sent.
b. PUTT/Weakfeed
The logic for a weakfeed terminal is not required for
the PUTT system. Because the PUTT system uses
underreaching relay(s) only for pilot trip keying, it is
impossible to apply this scheme to protect a system
which may have weakfeed condition.
c. POTT/ Weakfeed
For POTT and unblocking schemes, at the weak
source terminal, the Z3P/Z3G distance relays should
be set for reverse-looking, and the undervoltage
units (LVA, LVB, LVC) should be used. The basic
operating principle of the weakfeed trip logic for the
POTT and simplified unblocking scheme is as fol-
lows:
(1) Echo key for trip permission (Figure 3-31)
On internal faults, the strong source end
sends the trip (or unblocking) frequency sig-
nal to the weak end, and its pilot trip relay(s)
will trip, once it receives 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. With one
weakfeed condition, when the weak end
I.L. 40-385.1B
3-15
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