Refinements To Basic Schemes - GE L60 Instructions Manual

OVERVIEW CHAPTER 9: THEORY OF OPERATION
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.
Figure 9-14: Dual phase-comparison blocking scheme
There does not appear to be a good purpose for a three-frequency channel in dual phase-comparison blocking schemes
because the center frequency does not add to security or otherwise improve performance.
Note that a dual phase-comparison scheme using an ON-OFF channel has to be a combined blocking and tripping
scheme. During one polarity of half-cycle, it has to trip on absence of any received signal (blocking), and on the other
polarity of half-cycle it has to trip in the presence of the received signal.
In general, it can be concluded that dual phase-comparison can be accomplished in the blocking and in the tripping
modes. The overall performance of the scheme depends on the characteristics of the channel selected. While dual phase-
comparison reduces the maximum tripping time, it does so at the expense of simplicity and possibly some security
depending on how it is accomplished.
9

9.1.7 Refinements to basic schemes

There are a number of standard refinements that are required and normally included in all phase comparison schemes.
These are discussed in terms of the basic blocking scheme of Figure 9-4, but apply generally to all schemes, sometimes in
a different form.
9-22 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL