Multi-Terminal Lines; Charging Current Compensation - GE L60 Instruction Manual

OVERVIEW CHAPTER 9: THEORY OF OPERATION

9.1.8 Multi-terminal lines

To this point, the discussions have pertained principally to two-terminal lines. Phase-comparison schemes are often
applied to lines having more than two terminals and these applications differ depending on the channel equipment.
9.1.8.1 ON-OFF channel
The ON-OFF channel equipment is used in blocking type carrier schemes similar to that of Figure 9-16. Since this type of
scheme utilizes only one common frequency for all the transmitters and receivers, Figure 9-16 applies to multi-terminal
lines as well as two terminal lines. A blocking signal sent from any terminal is received at all the other terminals to provide
the necessary blocking via the single receiver at that terminal.
9.1.8.2 Frequency shift channel
Frequency-shift channels are generally used in tripping type schemes. Figure 1-17 illustrates a three-terminal line tripping
scheme using a frequency-shift channel. This arrangement requires two receivers at each terminal. One receiver is
required 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 one
receiver which would operate directly into AND1 without the need for AND5. Each channel has its own symmetry
adjustment.
Figure 9-18: Tripping scheme for three-terminal line

9.1.9 Charging current compensation

The premise for operation of phase-comparison protection schemes in general, and of the L60 phase comparison element
in particular, is that the sum of the currents entering the protected zone is zero. In the case of a power system transmission
line, this is not entirely true because of the capacitive charging current of the line. For short transmission lines, the
charging current is a small factor and can therefore be treated as an unknown error. In this application, the L60 can be
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deployed 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 can
be computed to provide increased sensitivity to fault current.
9-26 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL