10.5 UNDERSTANDING L60 OSCILLOGRAPHY
10 APPLICATION OF SETTINGS
10.5.2 TWO BREAKER CONFIGURATION
The 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 of
positive 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 10–11: BREAKER-AND-A-HALF SIGNAL PROCESSING
As shown in the oscillography, an external fault occurs on the breaker-and-a half diameter where the "F" CT/VT module is
fed from the Breaker 1 CT and the "M" CT/VT module is fed from the Breaker 2 CT. The positive and negative halves of the
waveform 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 of
currents is especially advantageous where fault currents are high and CT saturation is possible. Some times later, transient
blocking will recognize an external fault and operate blocking phase comparison until the fault current disappears.
L60 Line Phase Comparison System