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L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
OVERVIEW
CHAPTER 9: THEORY OF OPERATION
9
the selection between a blocking and a tripping scheme generally results 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 requires 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.
9.1.5.7 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 or cable company
can provide this link over microwave, cable, even pilot wires, or a combination of these. In such cases, the selection
between tripping and blocking schemes depends on the performance of the channel as specified. The same basic
schemes of the previous two figures apply.
9.1.5.8 Fiber optics
Fiber optic links are commonly used for protective relaying schemes. Since they propagate through the fiber rather than
over the power line, they are generally unaffected by faults and noise on the power system. Thus, with a fiber link there is
no problem getting a signal through the fault, so tripping type schemes are very acceptable. An exception occurs when the
fiber optic is embedded in the ground wire used on the line. In this case, the fault can be a result of a break in the ground
wire, which prevents transmission of the signals.
In any case, there is nothing about a fiber channel to alter the previous discussion concerning phase comparison
protection. The same schemes can be used, with the understanding that the fiber optic signal can be lost on occasion.
Phase comparison relaying schemes over fiber optic channels mostly are of the tripping types.
9.1.5.9 Conclusion of blocking versus tripping schemes
Implementations tend to be hybrids of blocking and tripping schemes. 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 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 needs to be made in conjunction with the chosen
channel and with a knowledge of the channel characteristics in the face of system noise. Many different combinations are
possible, but of these, only a selected few meet any given requirements.
9.1.6 Single versus dual phase comparison
In all the phase comparison schemes described so far, a trip attempt is made only every other half cycle. In the examples
illustrated, this was every positive half-cycle. Such schemes are termed single phase-comparison as against dual phase-
comparison where a trip attempt is made every half-cycle, positive and negative.
The only advantage of dual-comparison is that its maximum operating time to trip on internal faults is a half-cycle faster
than the maximum time for the single phase-comparison. The minimum times for both schemes is the same. This
difference in maximum time results because a fault can occur at such an instant in time when the current is just going
negative. Under such conditions, the single phase-comparison has to wait until the next positive half-cycle, while the dual
phase-comparison can trip on the upcoming negative half-cycle.
