CHAPTER 8: APPLICATION OF SETTINGS UNDERSTANDING L60 OSCILLOGRAPHYL60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 8-1588.5.2 Two-breaker configurationThe L60 has extra security when two CTs are brought into the relay individually and summed internally. Two currents areprocessed separately to derive the operating signal for each breaker, which is then used for fault detectors and theforming of positive and negative squares. This adds extra security for external faults beyond one of the breakers, withpossible CT saturation at the breaker carrying the full fault current from both the local and remote source, as shown in thefigure.Figure 8-11: Breaker-and-a-half signal processingAs shown in the oscillography, an external fault occurs on the breaker-and-a half diameter where the “F” CT/VT module isfed from the Breaker 1 CT and the “M” CT/VT module is fed from the Breaker 2 CT. The positive and negative halves of thewaveform are opposite at two CTs. Operating in tripping mode, the L60 detects this condition and transmits only whenpositive samples from both CTs are present. As a result, the transmit signal is very small and does not allow remoteoperation. In blocking mode, this fault results in a continuous block signal being sent to the remote relay. The separateprocessing of currents is especially advantageous where fault currents are high and CT saturation is possible. Sometimelater, transient blocking recognizes an external fault and operates blocking phase comparison until the fault currentdisappears.
