GE L60 Instruction Manual page 585

CHAPTER 9: THEORY OF OPERATION
With a frequency-shift transmitter-receiver as the communication equipment, the MARK block represents the receipt of
the hi-shift frequency and the SPACE block the low-shift frequency if the remote transmitter was keyed to high from a
positive current signal. The converse is true if the transmitter was keyed to high from a negative current signal. In any case,
the MARK block received at A, whatever it represents, corresponds to positive current at B while the SPACE block
corresponds to negative current at B.
If we consider an internal fault (as shown on the left side of the previous figure), the relay at A compares the modulated
quantities illustrated. If these two signals at terminal A were to be compared as shown in Figure 2A of the next figure over
a frequency-shift equipment, a trip output occurs if positive current and a receiver MARK signal are both concurrently and
continuously present for at least one-half cycle (8.33 ms at 60 Hz or 10 ms at 50 Hz). The trip output is continued for 18 ms
to ride over the following half cycle during which the current is negative, and the half cycle after that when the pick-up
timing takes place again.
Assuming that the MARK and SPACE signals cannot both be present concurrently then it can be argued that a comparison
can be made between the positive half-cycle of current and the absence of a receiver SPACE output. Figure 2B illustrates
this logic.
If the communication equipment happened to be a frequency shift channel so that both the MARK and the SPACE signals
were definite outputs, then Figure 2A represents a tripping scheme since tripping is predicated on the receipt of a remote
MARK or tripping signal. On the other hand, Figure 2B represents a blocking scheme in as much as it blocks tripping in the
presence of a MARK or blocking signal. It trips only in the absence of this signal.
The right side of the previous figure illustrates the conditions during an external fault. Referring to Figures 2A and 2B,
neither approach, the blocking or the tripping, results in a trip output for this condition since the AND circuits never
produce any outputs to the integrator.
The conditions illustrated in the previous figure are ideal. They seldom, if ever, occur in a real power system. Actually, an
internal fault does not produce a received signal MARK-SPACE relationship that is exactly in-phase with the locally
contrived single phase current. This is true for a variety of reasons including the following:
• Current transformer saturation
• Phase angle differences between the currents entering both ends of the line as a result of phase angle differences in
the driving system voltages
• Load and charging currents of the line
• Transit time of the communication signal
• Unsymmetrical build-up and tail-off times of the receiver
L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL
OVERVIEW
9
9-3