GE L60, Инструкция по эксплуатации, Страница: 374 / 498

9-6 L60 Line Phase Comparison Relay GE Multilin
9.2 DISTANCE BACKUP/SUPERVISION 9 APPLICATION OF SETTINGS
9
The L60 measures total harmonic distortion (THD) in all three phase currents and neutral current. These measurements
are available for protection purposes though FlexElements™ (universal comparators). The FlexElement™ output can be
used to block sensitive neutral instantaneous overcurrent or phase comparison on transformer energization.
Figure 9–3: USING FLEXELEMENTS™ FOR HARMONICS DETECTION
The 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 LOAD
Where tractional load is tapped form the line protected by L60, significant and variable negative-sequence current may
exist on the line, thereby not allowing sensitive FDL and FDH settings. On such lines, it is beneficial to detect faults based
on change in the sequence components of the currents. Again, FlexElements™ can be used for such an application.
Figure 9–4: USING FLEXELEMENTS™ TO DETECT CHANGES IN CURRENT SEQUENCE COMPONENTS
The example above depicts FlexElements™ programmed to detect changes in the positive, negative and zero-sequence
currents two times per power cycle on a 50 Hz system. The FlexElement™ outputs should be assigned to the
87PC FDL AUX
and
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 operating
quantities.