5.5 GROUPED ELEMENTS
In single phase comparison schemes, coincidence of the local and remote squares is detected during half the power cycle
only, positive or negative. As a result, some delay in operation can be expected under unfavorable fault inception. This
weakness of the single phase comparison schemes is eliminated in dual phase comparison schemes but cost of the com-
munication link is higher.
Some advantages of dual phase comparison and two frequency FSK PLC are incorporated in the unblocking scheme.
Since there is no third or guard frequency available, the PLC low frequency signal serves as the guard frequency for some
logic implemented in this scheme. Tripping is permitted if the FDL relay sees the change in received signal form low to high
(indicated that communication link is healthy and remote relay detected the fault) within 20 ms after fault is detected. If the
PLC low frequency has not being received prior the fault detection, the trip output is blocked as well. Another enhancement
of this scheme is the trip window defined by the
setting. This logic allows the relay to make a
87PC CHNL LOSS TRIP WINDOW
trip decision within this time if the PLC signal was lost in the course of the fault.
The phase comparison function can be used for three-terminal line protection and breaker-and-a-half configuration. The
feature combines the advantages of the modern digital relay with the traditional analog principle approach. Pulses received
from a PLC are digitally sampled at 64 samples per cycle, providing excellent resolution. This also eliminates carrier build-
ing-up and tailing-off problems, since the voltage threshold for received pulses is user-programmable. If a pulse received
from PLC is consciously distorted and is not equal to half of the sinewave, it can be adjusted with the
87PC CH1 ASYMMETRY
settings. All phase comparison signals are captured and available in oscillography for commis-
87PC CH2 ASYMMETRY
sioning, troubleshooting, and analysis purposes. The L60 features excellent stability during channel noise due to the high
sampling rate of the received signal, and the unique integrator makes the digital phase-comparison relay fully equivalent to
analogue phase-comparison relays.
The following figure illustrates the phase comparison logic. The choice of the scheme must made by protection and control
engineer according to the communication equipment employed, requirements of trip speed, and reliability. These schemes
are considered in Chapter 8: Theory of operation.
L60 Line Phase Comparison System