ABB RELION 611 Series Technical Manual page 226

Section 4
Protection functions
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Figure 105: Phase-segregated single busbar protection employing high-
impedance differential protection
Figure 106 shows an example for a system consisting of two busbar section coupled
with a bus coupler. Each busbar section consists of two feeders and both sections are
provided with a separate differential protection to form different zones. The formed
zones overlap at the bus coupler.
When the bus coupler is in the open position, each section of the busbar handles the
current flow independently, that is, the instantaneous incoming current is equal to the
total instantaneous outgoing current and the difference current is negligible. The
difference current is no longer zero with a fault in the busbar and the protection
operates.
With the bus coupler in the closed position, the current also flows from one busbar
section to another busbar section. Thus, the current flowing through the bus coupler
needs to be considered in calculating differential current. During normal condition,
the summation of the current on each bus section is zero. However, if there is a fault
1MRS757454 D
611 series
Technical Manual