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(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 (Figure 2-22) will satisfy
the NOT FORWARD, 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 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, 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 end high speed trips which
includes pilot trip, Zone1 trip, and high-set overcurrent trip, 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, and will also be disabled by
CIF Trip (CIF) (AND 49A).
c. Carrier Receiving Logic (Figure 2-22)
This logic includes OR-15 (not shown) and AND-44. Output "trip" (or unblocking) fre-
quency signal from the carrier receiver operates the logic OR-15 and will produce a
carrier trip (CR) signal from AND-44.
d. Channel Indicators (Figure 2-22)
The memorized Car Send (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 RX Ch1 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 prob-
lems 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 par-
allel 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 di-
rectional 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
with possible "tripping direction" polarity, which provides an electrical forward-
o
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 appli-
cation if SystType = POTT is set. For POTT application, the Zone3 (Z3FR) setting
†
Refer to Table 4-1 for Setting Cross-reference.
(10/94)
†
I.L. 40-386.3
2-15
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