9 THEORY OF OPERATION
In the case of ON-OFF power line carrier channels, the operating frequencies of the equipment at all terminals of the pro-
tected 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 dif-
ferent from all the others on the same line. For example, if the communication equipment in Figure 9-8 is of the frequency-
shift type, 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 transmit-
ters 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 could be simultaneously receiving a MARK signal from one and
a SPACE signal from another.
This would 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 may vary from about 1 to 10 watts and even more in extreme cases.
Consider an ON-OFF tripping type of scheme as defined by Figure 9-10. 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 will be supplying the bottom input to AND1, and tripping will take place when FDH operates to provide
the third input.
Figure 9–10: SINGLE-PHASE COMPARISON TRIPPING SCHEME PRINCIPLE
For external faults, the currents out of the mixing networks at the two ends of the line will be 180° out of phase with each
other. Therefore, during the half cycle that the SQ AMP at one end of the line is producing an output, the one at the remote
end is not, so no tripping will take place. It should be noted that a tripping type of scheme over an ON-OFF channel requires
transmitters of different frequency at each end of the line so that no receiver can receive the locally-transmitted signals; oth-
erwise tripping would occur during external faults. For this reason, such schemes are not generally applied.
L60 Line Phase Comparison System