GE L60 Instructions Manual page 667

CHAPTER 9: THEORY OF OPERATION
• Aside from the ability to accommodate continuous monitoring, the frequency-shift channel provides little advantage
over the ON-OFF carrier channel.
There are very few if any phase comparison tripping schemes in service over carrier channels mainly because of the fear
that it is not always be possible to get a trip signal through a fault.
Another scheme that has gained some favor is the unblocking scheme. It is a cross between blocking and tripping, in that
it operates in the blocking mode but the blocking signal is sent continuously even in the quiescent state (no fault), and so it
must be turned off in order to trip. Thus this scheme, as in the tripping schemes previously described, must include some
means to stop the blocking signal from being transmitted at an open terminal in order to permit tripping of the closed
remote terminal in the event of a fault. Here again the FDL logic of Figures 9-10 and 9-11 or the circuit breaker auxiliary
52/b switch can be used.
In general, unblocking utilizes frequency-shift channels because this permits monitoring of the continuous blocking
signals. As they are usually applied, ON-OFF channels do not lend themselves to monitoring because the single frequency
system transmits the same frequency from all transmitters and the loss of any one transmitter cannot be detected. If
applied in a normal duplex frequency basis (one in each direction), the ON-OFF channel provides the monitoring features at
the cost of carrier spectrum. However, this disadvantage can be overcome by the use of a new application of ON-OFF
equipment where the transmitters at the different terminals are operating at frequencies offset from each other yet close
enough to be nominally a single frequency system. This application permits monitoring, and at the same time has the
advantage of a higher channel speed than the frequency-shift channels, while utilizing less channel spectrum in three
terminal line applications.
9.1.5.5 Microwave links
Microwave links are commonly used for protective relaying including phase comparison schemes. However, because of
the high cost of the microwave equipment, the applications are generally limited to cases where a large number of control
and/or monitoring functions are needed between the same terminals as the relaying.
Since microwave links propagate through the atmosphere rather than the power line, they are generally unaffected by
faults and noise on the power system. Thus, with a microwave link there is no problem getting a signal through the fault, so
tripping type schemes are very acceptable. On the other hand, since there is a possibility of fading of the microwave signal,
there is some reluctance to use it in blocking schemes for fear of false tripping in the event of a fade during a nearby
external fault. Nevertheless, blocking schemes are used occasionally, mainly because the tripping scheme requires special
circuitry (as described earlier) in order to trip on single-end feed to a fault.
The communication equipment multiplexed on to a microwave system for protective relaying is invariably of the
frequency-shift type, and usually of the high speed variety. The previous two figures are representative of the tripping and
blocking schemes respectively. Since the microwave signal can fade, some of the frequency-shift receiver equipment
includes channel status detectors that operate into the relay logic to incapacitate all tripping when the channel conditions
are not normal. The ability to trip is then automatically reinstated when normality returns. With such an arrangement,
complete loss of receiver output incapacitates tripping. If the scheme were a blocking scheme similar to that of the Single-
Phase Comparison Blocking Scheme figure, complete loss of the channel during an external fault permits a false trip
unless an incapacitating feature is included in the scheme.
The receiver has only two outputs (high and low). Since the scheme trips on internal faults during the absence of the low-
shift output, and since the absence of both the low and high shift outputs incapacitates tripping (where used), the implied
requirement for tripping is the presence of the high-shift receiver output. While such a scheme is called a blocking scheme,
it appears to be, at least by implication, a tripping scheme.
In any case, there is nothing about a microwave channel to alter the previous discussion concerning phase comparison
protection. The same schemes can be used with the understanding that the microwave signal can fade on occasion. For
the most part, phase comparison relaying schemes over microwave channels have been of the tripping types.
9.1.5.6 Pilot wire links
There are few privately owned pilot wires that are used as a link in phase comparison schemes. Such applications require
frequency-shift communication equipment used in either a tripping or blocking mode as indicated in the previous two
figures. Aside from the considerations involved in tripping for a fault with single-end feed, which were discussed previously,
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
OVERVIEW
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