Most schemes that use an ON-OFF channel are arranged so that no transmission takes place during normal conditions (no
fault). This does not lend itself to continuous monitoring. However, schemes are available that periodically start transmis-
sion of a signal at one end of a line which, when received at the remote end, initiates a return transmitted signal. Such
schemes can be started manually or automatically on a time schedule. They are called carrier check-back schemes. They
can be arranged so as not to affect the normal operation of the scheme even in the event of a fault during a check-back
For the most part, phase comparison blocking carrier schemes use ON-OFF rather than frequency-shift channels, possibly
for one or more of the following reasons:
The overall speed of the protective scheme is directly related to the speed of the channel. Until recently high speed fre-
quency shift carrier channels were not available. Even today the ON-OFF channel is somewhat faster than the fastest
Noise at the input of an ON-OFF channel receiver would tend to produce a blocking signal output. Noise at the input of
a frequency-shift channel tends to drive its output to zero which is a tripping condition (in a blocking scheme). This
tends to make the frequency-shift blocking scheme less secure against false tripping during external faults. It is possi-
ble to build channel condition detectors (signal to noise, loss of channel, etc.) into frequency-shift channels and block
tripping when these detectors indicate trouble, but these features increase the complexity and the cost. This approach
tends to make the blocking scheme resemble the tripping scheme since the receiver must now indicate an intact chan-
nel in order to trip.
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 will not always be possible to get a trip signal through a fault.
Another scheme that has recently been gaining 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 aux-
iliary 52/b switch could 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 could not be detected. If applied in a
normal duplex frequency basis (one in each direction) the ON-OFF channel would provide 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 equip-
ment 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 appli-
e) MICROWAVE LINKS
Microwave links are quite 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 con-
trol and/or monitoring functions are needed between the same terminals as the relaying.
Since microwave links propagate through the atmosphere, rather than over the power line, they are generally unaffected by
faults and noise on the power system. Thus, with a microwave link there is no problem of 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 sig-
nal, 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. However, blocking schemes are used occasionally mainly because the tripping scheme requires special cir-
cuitry (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. Figures 9-11 and 9-12 are representative of the tripping and blocking
schemes respectively. Since, as mentioned above, 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 arrange-
L60 Line Phase Comparison System
9 THEORY OF OPERATION