GE L60 Instruction Manual

Also see for L60: Communications manual Communications guide Instruction manual Instruction manual Instruction manual

Contents

9-2 L60 Line Phase Comparison System GE Multilin9.1 OVERVIEW 9 THEORY OF OPERATION9It becomes apparent that a comparison such as that described above must be made on a single phase basis. That is, itwould 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 themagnitude 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 tocompare the phase angle or phase position of the currents is always the same. The left side of Figure 9–1 illustrates theconditions for a fault internal to the protected zone. The sketches show about 1 cycle of the currents under internal andexternal 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 thecommunication equipment happened to be a simple radio frequency transmitter-receiver, and if the positive half cycle ofcurrent keyed the transmitter to ON, then the MARK block corresponds to a received remote signal while the SPACE blockcorresponds to no signal. Conversely, if the negative portion of the current wave keyed the transmitter to ON, then theSPACE block would represent the received signal.With a frequency-shift transmitter-receiver as the communication equipment, the MARK block would represent the receiptof the hi-shift frequency and the SPACE block the low-shift frequency if the remote transmitter was keyed to high from apositive current signal. The converse would be true if the transmitter was keyed to high from a negative current signal. Inany case the MARK block received at A, whatever it represents, corresponds to positive current at B while the SPACEblock 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 modulatedquantities illustrated in the sketches. If these two signals at terminal A were to be compared as shown in Figure 9–2A overa 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 becontinued for 18 ms to ride over the following half cycle during which the current is negative, and the half cycle after thatwhen 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 9–2Billustrates this logic.If the communication equipment happened to be a frequency shift channel so that both the MARK and the SPACE signalswere definite outputs, Figure 9–2A would represent a tripping scheme since tripping is predicated on the receipt of a remoteMARK 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, neitherapproach, the blocking or the tripping, will result in a trip output for this condition since the AND circuits will never produceany 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 internalfault 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:1. Current transformer saturation.2. Phase angle differences between the currents entering both ends of the line as a result of phase angle differences inthe driving system voltages.3. Load and charging currents of the line.4. Transit time of the communication signal.5. Unsymmetrical build-up and tail-off times of the receiver.