GE L60 Instruction Manual page 603

CHAPTER 9: THEORY OF OPERATION
While, as a general rule, high-speed operation and security are opposite considerations, it is possible to design dual phase-
comparison schemes that can provide the added speed with little or no loss in security. However, these schemes are
somewhat more complex than equivalent single phase-comparison schemes. The following illustrates the dual phase-
comparison tripping scheme that is the counterpart of the single phase-comparison scheme. The differences are as
follows:
• The dual scheme uses two separate comparer integrator combinations, one for the positive half-cycle and the other
for the negative half-cycle.
• 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.
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.
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
Figure 9-13: Dual phase-comparison tripping scheme
OVERVIEW
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