9 THEORY OF OPERATION
The center frequency, while not actually used in the relay tripping logic, adds security to the scheme during transient condi-
The dual phase-comparison scheme of Figure 9-13 could be modified to operate over a two-frequency, frequency shift
channel by eliminating AND3, FDL, NOT1, and the center frequency. The transmitter could 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.
Figure 9-14 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 Figure 9-13 but probably is not as secure.
Figure 9–14: DUAL PHASE COMPARISON BLOCKING SCHEME
There does not appear to be any good purpose for a three-frequency channel in dual phase-comparison blocking schemes
since the center frequency would not add to the security, or otherwise improve the performance.
It is interesting to note that a dual phase-comparison scheme using an ON-OFF channel would have to be a combined
blocking and tripping scheme. During one polarity of half cycle, it would have to trip on absence of any received signal
(blocking), and on the other polarity of half cycle, it would have to trip in the presence of the received signal.
In general, it may be concluded that dual phase comparison may be accomplished in the blocking and in the tripping
modes. The overall performance of the scheme will be dependent on the characteristics of the channel selected. While dual
phase-comparison will reduce the maximum tripping time, it does so at the expense of simplicity and possibly some secu-
rity depending on how it is accomplished.
L60 Line Phase Comparison System