GE L60 Instruction Manual page 598

OVERVIEW CHAPTER 9: THEORY OF OPERATION
Figure 9-10: Single-phase comparison tripping scheme principle
For external faults, the currents out of the mixing networks at the two ends of the line are 180° out of phase with each
other. Therefore, during the half-cycle that the SQ AMP at one end of the line is producing an output, the one at the remote
end is not, so no tripping takes place. Note that a tripping-type of scheme over an ON-OFF channel requires transmitters of
different frequency at each end of the line so that no receiver can receive the locally-transmitted signals; otherwise
tripping occurs during external faults. For this reason, such schemes are not generally applied.
It appears that the tripping scheme as described has no need for an FDL function since no blocking coordination is
required as is in a blocking scheme. However, this is not the case. The FDL and NOT1 functions provide a means for tripping
when one end of the line is open as when picking up a faulted line from one end. For such a condition, the SQ AMP at the
open end receives no current and so produces no output to key its transmitter. Without a received signal the closed end of
the line cannot trip under any conditions even in the presence of a fault. The FDL function acts as a current detector. It is
set with a very low pick up so that any significant output from the mixing network causes it to produce a continuous
output. When the mixing network outputs goes to zero, FDL drops out causing an output from NOT1 which in turn keys the
9
transmitter on continuously. This is received at the remote end to provide a continuous signal at the bottom input to AND1.
Any fault that picks up FDH then is tripped at the closed end of the line.
If the mixing network includes a positive sequence output, then the load current keeps FDL picked up continuously. If the
mixing network includes only zero and/or negative sequence outputs, then the load current does not keep FDL picked up.
Thus, with zero or negative sequence phase comparison the receivers at both ends of the line produces outputs to AND1
continuously. When a fault occurs, FDL picks-up very fast to restore the keying function to SQ AMP. This operation
resembles a blocking scheme, although it is often called a permissive tripping scheme.
9-16 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL