GE L60 Instructions Manual page 663

CHAPTER 9: THEORY OF OPERATION OVERVIEW
Figure 9-9: Typical power line carrier arrangement
In the case of ON-OFF power line carrier channels, the operating frequencies of the equipment at all terminals of the
protected line are generally the same. Thus, a signal transmitted from any terminal is received at all terminals. This is not a
necessary requirement for using this kind of equipment. Rather it is desirable because the protection schemes that use
ON-OFF channels can accommodate a single frequency arrangement and this conserves the carrier spectrum.
When frequency-shift equipment is used over power line carrier, the frequencies of each transmitter on the line must be
different from all the others on the same line. For example, if the communication equipment in the Vector Relationships in
a Two-Terminal Faulted Line (A-to-G) figure is of the frequency-shift type, then the transmitter at the left end must operate
at the same frequencies as the receiver at the right end. Also, the right end transmitter and left end receiver must operate
at the same frequencies while the frequencies of the two transmitters must be different. This is necessary because with
frequency-shift equipment the transmitters associated with a given line protection scheme are not all generally sending
the MARK or the SPACE frequencies at the same time. Thus, if a receiver were able to receive more than one transmitter, it
can be simultaneously receiving a MARK signal from one and a SPACE signal from another.
This does not result in a workable protection scheme. When power line carrier channels are used, significant losses are
present in the coupling equipment and the line itself. Depending on these losses and the ambient noise on the line, the
transmitter power required can vary from about 1 to 10 watts and even more in extreme cases.
Consider an ON-OFF tripping type of scheme as defined by the following figure. For a moment assume that FDL and NOT1
do not exist in the logic. During an internal fault, the currents out of the mixing (or sequence) networks at both ends of the
line are in phase with each other so that the outputs of the SQ AMP are in phase at both ends of the line. The transmitters
at both ends of the line are keyed on during the same half cycles that their associated SQ AMPs are attempting to trip via
AND1. Thus, the receivers supply the bottom input to AND1, and tripping takes place when FDH operates to provide the
third input.
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L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-15