ABB MDAR Instruction Manual page 54

I.L. 40-385.5
FDGT has a range of 0 to 15 cycles, in 1 cycle steps.
It can be blocked by setting the FDGT to BLK.
The pilot distance unit PLTG is always active and has
the priority for tripping, therefore, practically, the
FDGT timer should be set to 3 cycles or longer for
security reason.
The pilot phase and/or pilot ground function(s) can
be disabled by selecting the PLTP and/or PLTG to
OUT. (note, the FDOG pilot trip function will also be
disabled when PLTG set to OUT).
POTT and simplified unblocking system includes the
following three portions:
(a) Tripping logic (Figure 27)
(1) For a forward external fault, the local pilot
PLTP and/or PLTG sees the fault, oper-
ates and keys.
The output from OR-40 will satisfy the
first input to the AND-30. Now, assuming
that TBM logic does not operate and PLT
pilot enable is set, three out of four
inputs of AND-30 are satisfied, but pilot
trip should not occur since the remote
transmitter still sends a guard (or block-
ing) frequency signal.
(2) For an internal fault, the pilot relays at
both ends, PLTP and/or PLTG, see the
internal fault and operate. Together with
received trip (or unblocking) frequency
signal CR via AND-44 in Figure 28, sat-
isfy AND-30 in Figure 27. 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 breaker trips.
(b) Carrier Keying Logic (Figure 28)
(1) Forward fault keying
For a forward internal or external fault,
the local pilot relay(s), PLTP and/or
PLTG, sees the fault, picks up, and oper-
ates OR-40, AND-45, OR-18 and AND-
35 if PLT pilot enable is set, and if func-
tional display "STYP" is selected to
POTT. The output signal from AND-35
will operate the reed relay SEND, key the
local transmitter and shift the transmit-
ting frequency from a guard to a trip for
10-2
POTT scheme (or from a blocking to an
unblocking for Unblocking scheme), to
allow the remote pilot relay system to
trip.
For security reasons, the keying circuit is
disabled by the time delay trip signal
TDT (includes Z2T, Z3T, CIFT AND GB),
which is not shown in Figure 28.
(2) Breaker open key logic — Echo keying
(Figure 28)
Since the POTT and the simplified
unblocking system require the receiving
of a permissive signal from the remote
end for pilot trip, provision should be
made for covering the condition when
the remote breaker is opened.
MDAR (REL-300) version 2.10 uses
echo key approach (note, some older
version use 52b key approach) for
breaker open key condition. When the
remote breaker is opened, the remote
breaker echo key logic will echo the sig-
nal back to the local terminal for permis-
sive tripping. The echo key logic AND-
34B, in Figure 28, will be satisfied if the
52b contact is closed (breaker open),
and neither forward pilot relay (OR-40)
nor reverse looking relay (for reverse
block) has output, and there is output
from AND-44.
The echo key logic will operate the OR-
18 and send carrier for 150 ms. The 8/0
ms delay between AND-44 and AND-
34B is required for overriding any noise
from the channel.
(3) Signal Continuation Logic (Figure 28)
This logic includes the signal of TRSL, 0/
150 ms timer, AND-34A, OR-18 and
AND-35. The 0/150 ms signal continua-
tion time is required to keep the local
transmitter at the trip frequency (or
unblocking) for 150 ms immediately after
the local breaker trip signal TRSL is
present. This is required for sequential
trip. This logic will be disabled by the
time delay trip signal TDT via 0/300 ms
timer and AND-34A, i.e. the signal
continuation logic will not be performed
on any time delay trip operation. Also,