Two-Breaker Configuration - GE L60 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS UNDERSTANDING L60 OSCILLOGRAPHY

8.5.2 Two-breaker configuration

The L60 has extra security when two CTs are brought into the relay individually and summed internally. Two currents are
processed 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 saturation at the breaker carrying the full fault current from both the local and remote source, as shown in the
figure.
Figure 8-11: 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
positive 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 results in a continuous block signal being sent to the remote relay. The separate
processing of currents is especially advantageous where fault currents are high and CT saturation is possible. Sometime
later, transient blocking recognizes an external fault and operates blocking phase comparison until the fault current
disappears.
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L60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 8-15