CHAPTER 9: THEORY OF OPERATION
OVERVIEW
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
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9
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 can be out of service but in the closed position. This requires a
bypass of its 52/b switch that is open.
Regardless of the tripping scheme used, it is obvious from the Typical Power Line Carrier Arrangement figure 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, then no tripping can take place. The amount of attenuation in signal
that is produced by the fault depends on the type of coupling (single phase, interphase, and so on), 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.
The following figure illustrates the same tripping scheme as the previous figure except that it utilizes a frequency shift
rather than an ON-OFF communication set. 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 unaffected 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.
Figure 9-11: Single-phase comparison tripping scheme
