It becomes apparent that a comparison such as that described above must be made on a single phase basis. That is, it
would not be possible to compare all three phase currents at terminal A individually with all three at terminal B over one sin-
gle channel and one single comparing unit. However, to reduce communications channel requirements, all three phase cur-
rents are mixed to produce a single phase quantity whose magnitude and phase angle have a definite relation to the
magnitude and phase angle of the three original currents. It is this single phase quantity that is phase compared with a sim-
ilarly obtained quantity at the remote end(s) of the line.
While there are many variations on the basic scheme (these are discussed subsequently), the general method employed to
compare the phase angle or phase position of the currents is always the same. The left side of Figure 9–1 illustrates the
conditions for a fault internal to the protected zone. The sketches show about 1 cycle of the currents under internal and
external faults to represent relay 'A' trip logic.
The MARK-SPACE designations given to the received signal are for identification and have no special significance. If the
communication equipment happened to be a simple radio frequency transmitter-receiver, and if the positive half cycle of
current keyed the transmitter to ON, then the MARK block corresponds to a received remote signal while the SPACE block
corresponds to no signal. Conversely, if the negative portion of the current wave keyed the transmitter to ON, then the
SPACE block would represent the received signal.
With a frequency-shift transmitter-receiver as the communication equipment, the MARK block would represent 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 would be 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 Figure 9–1), the relay at A would be comparing modulated
quantities illustrated in the sketches. If these two signals at terminal A were to be compared as shown in Figure 9–2A over
a frequency-shift equipment, a trip output would occur if positive current and a receiver MARK signal were both concur-
rently and continuously present for at least one-half cycle (8.33 ms at 60 Hz or 10 ms at 50 Hz). The trip output would be
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 might be argued that a compari-
son could be made between the positive half cycle of current and the absence of a receiver SPACE output. Figure 9–2B
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, Figure 9–2A would represent a tripping scheme since tripping is predicated on the receipt of a remote
MARK or tripping signal. On the other hand, Figure 9–2B would represent a blocking scheme in as much as it will block trip-
ping in the presence of a MARK or blocking signal. It will trip only in the absence of this signal.
The right side of Figure 9–1 illustrates the conditions during an external fault. Referring to Figures 9–2A and 9–2B, neither
approach, the blocking or the tripping, will result in a trip output for this condition since the AND circuits will never produce
any outputs to the integrator.
The conditions illustrated in Figure 9–1 are ideal. They seldom, if ever, occur in a real power system. Actually, an internal
fault would not produce a received signal MARK-SPACE relationship that is exactly in phase with the locally contrived sin-
gle 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
9 THEORY OF OPERATION