ABB REL300 Instruction Manual page 61

AND-35 will operate the reed relay (CARSND), key the local transmitter, shift the
transmitting frequency from guard to trip (or from a blocking to an unblocking), to al-
low the remote pilot relay system to trip.
(2) Echo Keying
Since the POTT and the Simplified Unblocking schemes 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.
When the remote breaker is opened, the inputs of AND34B will satisfy the NOT FOR-
WARD, NOT REVERSE and 52b=1 conditions. Any carrier RCVR (CR) will produce
an output from AND 34B and also a SEND signal from AND 35 via OR-18. This echo
keying will be stopped by itself after 150 ms due to the input timer of AND 34B.
(3) Signal Continuation
This logic includes the signal of TRSL, 0/150 ms timer, AND-34A, OR-18, and AND-
35. The 0/150 ms signal continuation time is required to keep the local transmitter at
the trip frequency (or unblocking) for 150 ms after the local breaker trip signal TRSL
is present, in case of sequential trip on the system. This logic will be disabled by the
signal via TDT, the 0 / 300 ms timer (AND-34A) on any time delay trip operation. The
signal continuation logic will not perform if the trip is caused by CIF trip (AND 49A).
c. Carrier Receiving Logic (Figure 3-23, page 3-43)
This logic includes OR-15 (not shown) and AND-44. Output "trip" (or unblocking) frequency
signal from the carrier receiver operates the logic OR-15 and will produce a carrier trip (CR)
signal from AND-44.
d. Channel Indicators
The memorized SEND indicator will be displayed after the breaker trips and the frequency
shifts to trip 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 involved, 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 direc-
tional relay upon clearing of a fault in the adjacent (but not parallel) line. When the adjacent
line breaker trips, it interrupts 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
ping direction" polarity, which provides an electrical forward-torque to the ground directional
relay. Therefore, it is desired 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 ap-
I.L. 40-385.7
with possible "trip-
o
3-17