ABB REL-300 Instruction Leaflet page 58

For a reverse fault, both the local carrier start
relay(s) and the remote pilot relay(s) see the
fault and operate. The local carrier start
relay(s) start the carrier and send a blocking
signal to block the remote pilot relay from
tripping. After the fault is cleared by the ex-
ternal breaker, the remote breaker may have
a tendency to trip falsely if the carrier start
unit resets faster than the pilot trip unit. The
0/50 ms timer between the OR-41 and AND-
51 holds the carrier signal for 50 ms after the
carrier start units have been reset for improv-
ing this problem. This logic also provides
transient block and unblock (TBM) effect on
power reversal.
The subsequent out-of-step condition (as de-
scribed in Section 3.4.17.1), may cause the
reverse looking units to fail to operate on ex-
ternal faults, and introduce false pilot tripping
at the other end. Enhanced logic has been
added to the design as shown in Figure 3-25,
which includes OR-41C, 32/0 ms timer,
AND-41B and OR-41. It utilizes the not
FDOP (or FDOG) and LV condition (LV units
set at 40 to 60 vitalist initiate the TBM circuit;
and sends a blocking signal to the remote
end. Set OSB to YES for supervising AND-
41B when this enhanced logic is required in
the application. Set WFEN to YES if this ter-
minal may become a weakfeed condition.
(3) Internal fault preference and squelch
On a close-in fault, the carrier start unit may
operate and start the transmitter. This opera-
tion may block the system from pilot tripping.
The negating signal from OR-16 to AND-50
will provide an internal fault preference fea-
ture for solving this problem. The squelch 0/
150 ms timer is required for improving the
problem if the local breaker tripped faster
than the remote breaker on an internal fault.
The logic holds the carrier key circuit on the
"stop" mode for 150 ms after any high speed
tripping, including pilot trip, Zone 1 trip and
instantaneous overcurrent trip.
(5/92)
c. Carrier Receiving Logic (Figure 3-25)
Carrier signal from the receiver output will be directly
applied to AND-47 to disable the pilot tripping func-
tion.
d. Channel Indication (not shown in Figure 3-25)
Since the carrier channel turns "ON" for external
faults only, the channel indicators (SEND and
RCVR) should not be sealed-in.
e. Channel Simulation
Same as for POTT scheme.
f. Programmable Reclosing Initiation (Figure 3-19)
Same as for POTT scheme.
3.5.4 POTT or Simplified Unblocking Weakfeed
(see 3.10.3)
3.6 PILOT GROUND
Pilot ground is more dependable on high resistance
faults because it is supplemented with FDOG and
IOM (refer to Figure 3-26).
Pilot ground is more secure on POTT/unblocking
schemes on some special power system conditions,
such as shown in Figure 3-27. A φφG fault is on the
paralleled line section. Due to the system condition,
fault current flows in the protected line would be
I1+I2 from A to B, and Io from B to A. The operation
of pilot distance relays would be a phase relay at A
and a ground relay at B. The result would be errone-
ous directional comparison of an external fault as an
"internal" one. The POTT/unblocking scheme will in-
correctly trip out of the protected line.
MDAR POTT/Unblocking pilot ground unit (PLTG/
FDOG) is supervised by the reverse-looking ground
unit (RDOG). The "Reverse-Block" logic is as shown
in Figure 3-32. At terminal A, the RDOG disables the
PLTG/FDOG trip/key functions via AND-35 and
AND-30. At terminal B, it will receive no carrier signal
for permissive trip. The reverse-block logic also pro-
vides the conventional TBM feature to prevent false
operation on power reversal. It should be noted that
a "Block-the-Block" logic is also included in the cir-
cuit, as shown in Figure 3-32. The Block-the-Block
logic is to prevent the Reverse-Block logic from over-
blocking (see the following system condition). If the
breaker is unequal-pole closing on a φG fault, say
I.L. 40-385.1B
3-13