9-6 L60 Line Phase Comparison System GE Multilin9.2 DISTANCE BACKUP/SUPERVISION 9 APPLICATION OF SETTINGS9The L60 measures total harmonic distortion (THD) in all three phase currents and neutral current. These measurementsare available for protection purposes though FlexElements™ (universal comparators). The FlexElement™ output can beused to block sensitive neutral instantaneous overcurrent or phase comparison on transformer energization.Figure 9–4: USING FLEXELEMENTS™ FOR HARMONICS DETECTIONThe typical pickup setting for THD is 10 to 25%. THD measurements are available per source. As such, for breaker-and-a-half applications, the source used to sum the CT currents can be used.9.2.6 TRACTIONAL LOADWhere tractional load is tapped form the line protected by L60, significant and variable negative-sequence current mayexist on the line, thereby not allowing sensitive FDL and FDH settings. On such lines, it is beneficial to detect faults basedon change in the sequence components of the currents. Again, FlexElements™ can be used for such an application.Figure 9–5: USING FLEXELEMENTS™ TO DETECT CHANGES IN CURRENT SEQUENCE COMPONENTSThe example above depicts FlexElements™ programmed to detect changes in the positive, negative and zero-sequencecurrents two times per power cycle on a 50 Hz system. The FlexElement™ outputs should be assigned to the 87PC FDL AUXand 87PC FDH AUX settings to start phase comparison when a change in the current components is detected. These Flex-Elements™ are immune to slow increases of the current components, but will operate for step changes of the operatingquantities.
