5-198 L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUALGROUPED ELEMENTS CHAPTER 5: SETTINGS5In single phase comparison schemes, coincidence of the local and remote squares is detected during half the power cycleonly, positive or negative. As a result, some delay in operation can be expected under unfavorable fault inception. Thisweakness of the single phase comparison schemes is eliminated in dual phase comparison schemes but cost of thecommunication link is higher.Some advantages of dual phase comparison and two frequency FSK PLC are incorporated in the unblocking scheme. Sincethere is no third or guard frequency available, the PLC low frequency signal serves as the guard frequency for some logicimplemented 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. Ifthe PLC low frequency has not being received prior the fault detection, the trip output is blocked as well. Anotherenhancement of this scheme is the trip window defined by the 87PC CHNL LOSS TRIP WINDOW setting. This logic allows therelay to make a 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. Thefeature combines the advantages of the modern digital relay with the traditional analog principle approach. Pulsesreceived from a PLC are digitally sampled at 64 samples per cycle, providing excellent resolution. This also eliminatescarrier building-up and tailing-off problems, since the voltage threshold for received pulses is user-programmable. If apulse received from PLC is consciously distorted and is not equal to half of the sinewave, it can be adjusted with the 87PCCH1 ASYMMETRY and 87PC CH2 ASYMMETRY settings. All phase comparison signals are captured and available inoscillography for commissioning, troubleshooting, and analysis purposes. The L60 features excellent stability duringchannel 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 controlengineer according to the communication equipment employed, requirements of trip speed, and reliability.
