9-14 L60 Line Phase Comparison Relay GE Multilin9.5 UNDERSTANDING L60 OSCILLOGRAPHY 9 APPLICATION OF SETTINGS99.5.2 TWO BREAKER CONFIGURATIONThe L60 has extra security when 2 CTs are brought into the relay individually and summed internally. Two currents are pro-cessed separately to derive the operating signal for each breaker, which is then used for fault detectors and the forming ofpositive and negative squares. This adds extra security for external faults beyond one of the breakers, with possible CT sat-uration at the breaker carrying the full fault current from both the local and remote source as illustrated below.Figure 9–11: 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 when pos-itive samples from both CTs are present. As a result, the transmit signal is very small and does not allow remote operation.In blocking mode, this fault would result in a continuous block signal sent to the remote relay. The separate processing ofcurrents is especially advantageous where fault currents are high and CT saturation is possible. Some times later, transientblocking will recognize an external fault and operate blocking phase comparison until the fault current disappears.
