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L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
OVERVIEW
CHAPTER 9: THEORY OF OPERATION
9
•
A three-frequency, frequency-shift channel is used in dual phase comparison. The high-shift operates in conjunction
with the positive half-cycle while the low-shift works with the negative half-cycle. When the channel is not keyed to
either high or low, it operates on the center frequency. There is no center frequency output from the receiver into the
relay tripping logic.
•
AND3 is included to make it impossible to key both frequencies simultaneously. It also gives preference to the low-shift
that is sent continuously when FDL is dropped out. Thus, on single-end feed tripping can take place only on the
negative half-cycle.
Figure 9-13: Dual phase-comparison tripping scheme
The center frequency, while not actually used in the relay tripping logic, adds security to the scheme during transient
conditions.
This dual phase-comparison scheme can be modified to operate over a two-frequency, frequency-shift channel by
eliminating AND3, FDL, NOT1, and the center frequency. The transmitter can then be arranged to send the low-shift
frequency continuously except when keyed by the positive SQ. AMP to the high shift frequency. This arrangement, though
simpler than the three-frequency scheme, is deemed to be less secure.
The following illustrates a dual phase-comparison blocking scheme using a two-frequency, frequency-shift channel. Since
one or the other of the two frequencies must be on at all times, and since both are blocking frequencies, there appears to
be little need for an FDL function. Thus, it is not included. When the transmitter is not keyed, it sends low-shift continuously
and when it is keyed by the negative squaring amplifier, it shifts to high for the negative half-cycle. This scheme is simpler
than that of the previous scheme but probably is not as secure.
Содержание L60
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Страница 498: ...7 10 L60 LINE PHASE COMPARISON SYSTEM INSTRUCTION MANUAL TARGETS MENU CHAPTER 7 COMMANDS AND TARGETS 7 ...
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