ABB 615 series Technical Manual page 863

1MAC059074-MB A
615 series ANSI
Technical Manual
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Figure 443: Broken secondary detection by MCS-A, MCS-B, MCS-C
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 87A, 87B, 87C and MCS-A, MCS-B, MCS-
C consider the current as an increased differential or unbalance current because of the
broken CT wire in phase C. Both 87A, 87B, 87C and MCS-A, MCS-B, MCS-C receive
the differential current of approximately 1.0 pu The main differential protection 87A,
87B, 87C cannot trip because of the higher current setting.
All CTs must have the same ratio.
Section 6
Supervision functions
857