GE L60 Instruction Manual page 600
Line phase comparison system
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Also See for L60:
- Instructions manual (768 pages) ,
- Instruction manual (736 pages) ,
- Communications manual (532 pages)
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OVERVIEW
The Single-Phase Comparison Blocking Scheme Principle figure and the following figure illustrate phase comparison
blocking schemes with ON-OFF and frequency-shift channels respectively. Figure 1-5 was discussed earlier and Figure 1-
12 is exactly the same except for the high frequency shift which is not used in the protection scheme. While only one of the
two frequencies of the frequency-shift equipment is used in the protection scheme, the second frequency does perform a
useful function. It provides a means for continuous monitoring of the channel. Since one of the two frequencies is always
being transmitted, it is possible to monitor the signal at each receiver continuously and incapacitate the protective scheme
and/or provide indication at that terminal if the signal is lost.
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
transmission 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 operation.
Mostly, 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
frequency shift carrier channels were not available. Even today the ON-OFF channel is somewhat faster than the
fastest frequency-shift channel.
•
Noise at the input of an ON-OFF channel receiver tends to produce a blocking signal output. Noise at the input of a
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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
possible to build channel condition detectors (signal to noise, loss of channel, and so on) 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 channel in order to trip.
9-18
Figure 9-12: Single-phase comparison blocking scheme
CHAPTER 9: THEORY OF OPERATION
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
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