9 THEORY OF OPERATION
ment, complete loss of receiver output would incapacitate tripping. If the scheme were a blocking scheme similar to that of
Figure 9-12, complete loss of channel during an external fault would permit a false trip unless an incapacitating feature
were 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 basic schemes may 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.
f) PILOT WIRE LINKS
There are few, if any, privately owned pilot wires that are used as a link in phase comparison schemes. However, such
applications would require a frequency-shift communication equipment used in either a tripping or blocking mode as indi-
cated in Figures 9-11 and 9-12 respectively. Aside from the considerations involved in tripping for a fault with single-end
feed, which were discussed previously, the selection between a blocking and a tripping scheme will generally result from a
compromise between security and reliability. In order to make such a selection, consideration of the pilot pair, its protection,
and its physical location in relation to power conductors must be evaluated.
In general, a high speed channel would require pilot wires that have a frequency response that is somewhat better than the
standard telephone voice circuits.
Possibly because of the uncertainties of channel characteristics, plus the availability of pilot wire relays that are much lower
in overall cost, phase comparison over privately owned pilot wires is not a common application.
g) LEASED (TELEPHONE COMPANY) FACILITIES
There has been some use of phase comparison relaying over leased facilities including voice grade pilot wire circuits. In
general, if a customer requires or specifies the characteristics of a leased channel, the local telephone company could pro-
vide this link over microwave, cable, even pilot wires, or a combination of these. In such cases the selection between trip-
ping: and blocking schemes will depend on the performance of the channel as specified. The same basic schemes of
Figures 9-11 and 9-12 would apply.
h) FIBER OPTICS
Fiber optic communications links are quite commonly used for protective relaying schemes. Since fiber optic links propa-
gate through the fiber, rather than over the power line, they are generally unaffected by faults and noise on the power sys-
tem. Thus, with a fiber link there is no problem of getting a signal through the fault, so tripping type schemes are very
acceptable. An exception may occur when the fiber optic is embedded in the ground wire used on the line. In this case, the
fault may be a result of a break in the ground wire which would prevent transmission of the signals.
In any case, there is nothing about a fiber channel to alter the previous discussion concerning phase comparison protec-
tion. The same basic schemes may be used with the understanding that the fiber optic signal can be lost on occasion. For
the most part, phase comparison relaying schemes over fiber optic channels are of the tripping types.
i) SUMMATION OF BLOCKING VS. TRIPPING SCHEMES
The foregoing discussion of blocking and tripping schemes was presented without the benefit of a concise definition of
these terms. As indicated in the discussion, the difficulty of making such definitions which would always apply is brought
about by the channel status feature used in some frequency-shift blocking schemes. Such arrangements tend to be
hybrids. Thus, the following simple definitions exclude any considerations of channel status features:
A blocking scheme is one that requires a specific output signal from the associated receiver in order to block tripping.
Tripping can only take place during the time that this signal is absent.
A tripping scheme is one that requires a specific output signal from the associated receiver in order to permit tripping.
Tripping can only take place during the time that this signal is present.
Where channel status logic is used, these definitions will have to be modified to meet the exact logic of the scheme.
In general, the selection of a blocking or a tripping scheme is one that should be made in conjunction with the chosen chan-
nel and with a knowledge of the channel characteristics in the face of system noise. Many different combinations are possi-
ble, but of these, only a selected few will meet any given set of requirements.
L60 Line Phase Comparison System