3-Terminal Line Application; Weakfeed Trip Application - ABB REL 301 Instruction Leaflet

2.5.5

3-terminal Line Application

For Blocking 3-terminal line applications, since the frequency of the 3 transmitters are the same,
any one transmitter starting will block the pilot system from tripping, therefore, logic for the 3-ter-
minal pilot system would be the same as that used for the 2-terminal system. However, for
POTT/Unblock and PUTT systems, since the transmitter frequencies are different at each ter-
minal, logic for the second receiver (RCVR-2) is added to the system when the application in-
volves 3-terminal lines. Setting "3-Term." should be set to "YES" when a 3-terminal line system
is required.
a. Additional Logic For POTT and Simplified Unblocking (Figure 2-28)
This logic includes a contact converter (CC) for RCVR-2, AND 55, and logic for the second
receiver indication (not shown). Voltage applied to RCVR-2 operates the contact converter
and produces the channel receive signal (CR) from AND 63A via AND 55 and AND 64
which allows pilot tripping (Figure 2-22, OR 21).
b. Additional Logic for PUTT (Figure 2-29)
The additional logic for PUTT is similar to that described for POTT scheme, except logic
includes AND 56, AND 57 and 50/0 millisecond timer. Since Zone-1 reach dictates trans-
mitting of the permissive signal, the fault could possibly be detected by only one remote
terminal. For a close-in Zone-1 fault, only the local terminal can key its transmitter and the
other two may not. This logic provides a CR pilot trip signal for 50 ms for, system security,
if either channel is received. For a fault which is detected by relays at both remote terminals,
AND 55 logic will not be satisfied, then channel (CR) will be performed via the logic which
allows pilot tripping (Figure 2-22, OR 21).
2.5.6

Weakfeed Trip Application

a. Block/Weakfeed
Special logic for a weakfeed terminal is not required for Blocking systems since Blocking
systems 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: For the case of "OS Block" is set to YES, Weakfeed should be set to YES if this
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 not necessary to apply
weakfeed logic.
c. POTT/ Weakfeed
For POTT and unblocking schemes, at the weak source terminal, the Zone-3Ø /Z3G dis-
tance 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 follows:
1) Echo key for trip permission (Figure 2-30)
On internal faults, the strong terminal(s) send the permissive (or unblocking) frequency sig-
nal to the weak terminal, and the strong terminal(s) pilot trip logic will trip, once echo trip
terminal can become a weak terminal (e.g. certain system configurations). Refer
to Figure 2-24, logic AND 41B and OR 41C.
I.L. 40-386.4
2-21