![GE Digital Energy L60 Instruction Manual Download Page 535](http://html.mh-extra.com/html/ge-digital-energy/l60/l60_instruction-manual_571854535.webp)
CHAPTER 9: THEORY OF OPERATION
OVERVIEW
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
9-21
9
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.
9.1.7 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.
9.1.7.1 Symmetry adjustment
As noted in a previous section, receivers are not always symmetrical in their response. That is, if a transmitter is keyed on
and off symmetrically every half-cycle, the remote receiver output does not necessarily correspond exactly to the keying
signal. For example, if an ON-OFF transmitter were keyed on for a half-cycle and then off for a half-cycle, and so on, the
remote receiver output can be on for more than a half-cycle and off for less than a half-cycle. This affect is primarily due to
the filter response in the receiver and is common with ON-OFF type of equipment. It is not a constant value but rather
depends on operating frequencies as well as received signal strength. Thus, this asymmetry can vary from equipment to
equipment and from time to time (as atmospheric conditions change) in service.
Frequency shift channels are generally symmetrical in their response when the discriminator in the receiver is balanced. If
the discriminator is biased to one side or the other, the receiver output tends to favor the side to which it is biased.