Figure 478: Broken secondary detection by HZCCxSPVC
In the example, the incoming feeder is carrying a load of 2.0 pu and both outgoing
feeders carry an equal load of 1.0 pu However, both HIxPDIF and HZCCxSPVC
consider the current as an increased differential or unbalance current because of
the broken CT wire in phase C. Both HIxPDIF and HZCCxSPVC receive the differential
current of approximately 1.0 pu The main differential protection HIxPDIF cannot
operate because of the higher current setting.
All CTs must have the same ratio.
The ALARM output of the CT supervision function can be used to energize an
auxiliary relay which can short-circuit the current CT wires, making the busbar
differential protection inoperative. This arrangement does not prevent unwanted
operation of HIxPDIF if the start setting is below the rated load. For example, if the
Supervision functions
1MRS757644 H
894 620 series
Technical Manual
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