9-26 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUALOVERVIEW CHAPTER 9: THEORY OF OPERATION9Benefits of this scheme are that operating time is faster compared with single phase-comparison with enough securitybuilt into the scheme.9.1.8 Multi-terminal linesTo this point, the discussions have pertained principally to two-terminal lines. Phase-comparison schemes are oftenapplied to lines having more than two terminals and these applications differ depending on the channel equipment.9.1.8.1 ON-OFF channelThe ON-OFF channel equipment is used in blocking type carrier schemes similar to that of Figure 9-16. Since this type ofscheme utilizes only one common frequency for all the transmitters and receivers, Figure 9-16 applies to multi-terminallines as well as two terminal lines. A blocking signal sent from any terminal is received at all the other terminals to providethe necessary blocking via the single receiver at that terminal.9.1.8.2 Frequency shift channelFrequency-shift channels are generally used in tripping type schemes. Figure 1-17 illustrates a three-terminal line trippingscheme using a frequency-shift channel. This arrangement requires two receivers at each terminal. One receiver isrequired for each remote transmitter because each transmitter is operated at a different frequency. In order to trip, a high-shift output is required from both receivers concurrently to AND5. A two-terminal line scheme would require only onereceiver which would operate directly into AND1 without the need for AND5. Each channel has its own symmetryadjustment.Figure 9-18: Tripping scheme for three-terminal line9.1.9 Charging current compensationThe premise for operation of phase-comparison protection schemes in general, and of the L60 phase comparison elementin particular, is that the sum of the currents entering the protected zone is zero. In the case of a power system transmissionline, this is not entirely true because of the capacitive charging current of the line. For short transmission lines, thecharging current is a small factor and can therefore be treated as an unknown error. In this application, the L60 can bedeployed without voltage sensors and the line charging current is included as a constant term in the total variance,increasing the differential restraint current. For long transmission lines the charging current is a significant factor, and canbe computed to provide increased sensitivity to fault current.
