It appears that the tripping scheme as described above has no need for an FDL function since no blocking coordination is
required as is in a blocking scheme. However, this is not the case. The FDL and NOT1 functions provide a means for trip-
ping when one end of the line is open as when picking up a faulted line from one end. For such a condition, the SQ AMP at
the open end receives no current and so produces no output to key its transmitter. Without a received signal the closed end
of the line cannot trip under any conditions even in the presence of a fault. The FDL function acts as a current detector. It is
set with a very low pick up so that any significant output from the mixing network causes it to produce a continuous output.
When the mixing network outputs goes to zero, FDL drops out causing an output from NOT1 which in turn keys the trans-
mitter on continuously. This is received at the remote end to provide a continuous signal at the bottom input to AND1. Any
fault that picks up FDH will then be tripped at the closed end of the line.
If the mixing network includes a positive sequence output, load current will keep FDL picked up continuously. If the mixing
network includes only zero and/or negative sequence outputs, load current will not keep FDL picked up. Thus, with zero or
negative sequence phase comparison the receivers at both ends of the line will be producing outputs to AND1 continu-
ously. When a fault occurs, FDL picks-up very fast to restore the keying function to SQ AMP. This operation resembles a
blocking scheme, although it is often called a permissive tripping scheme.
Another scheme to facilitate tripping on single end feed, uses a circuit breaker 52/b switch rather than FDL and NOT1.
When the breaker is open, the 52/b switch closes and keys the associated transmitter on continuously. When the breaker is
closed, the 52/b switch is open and keying is under control of the SQ AMP. While on the surface the use of 52/b appears
simple and direct, the following problems arise that can require more complex logic and station wiring:
The 52/b contacts do not generally operate in synchronism with the main poles of the breaker so some timing functions
must be included with the logic to compensate for this.
In multi-breaker schemes, such as ring buses, two breakers at each terminal are associated with each line so 52/b
switches from each breaker are required in series.
In multi-breaker schemes one of the two breakers may be out of service but in the closed position. This would require
a bypass of its 52/b switch which is open.
Regardless of which tripping scheme is used, it is obvious from Figure 9-9 that in order to trip either circuit breaker A or B
for an internal fault at P it is necessary to get a carrier signal through the fault. If the fault attenuates the signal so that this
does not happen, no tripping can take place. The amount of attenuation in signal that is produced by the fault will depend
on the type of coupling (single phase, interphase, etc.), the type of fault, the phase involved, and the location of the fault on
the line. The evaluation of these factors is outside the scope of this discussion.
Figure 9-11 illustrates the same tripping scheme as Figure 9-10 except that it utilizes a frequency shift rather than an ON-
OFF communication set. The same comments apply to this scheme as do to that of Figure 9-9. A tripping scheme that
operates over a power line carrier channel runs the risk of a failure to trip on internal faults because of signal attenuation.
During external faults the line traps isolate the signal on the protected line from the fault. This is of no significance because
attenuation or loss of signal on external faults cannot result in any maloperations. Conversely, a blocking scheme is unaf-
fected by loss or attenuation of signal during internal faults because absence of a signal is required in order to trip. During
external faults it is important that the blocking signal be isolated from the fault because loss of the signal can result in a
false trip. The line traps provide this isolation.
Figures 9-5 and 9-12 illustrate phase comparison blocking schemes with ON-OFF and frequency-shift channels respec-
tively. Figure 9-5 was discussed earlier and Figure 9-12 is exactly the same except for the high frequency shift which is not
used in the protection scheme. While only one of the two frequencies of the frequency-shift equipment is used in the protec-
tion scheme, the second frequency does perform a useful function. It provides a means for continuous monitoring of the
channel. Since one of the two frequencies is always being transmitted, it is possible to monitor the signal at each receiver
continuously and incapacitate the protective scheme and/or provide indication at that terminal if the signal is lost.
L60 Line Phase Comparison System
9 THEORY OF OPERATION