ABB RELION REX640 Technical Manual page 780

Protection and control

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Protection functions

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

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 in any busbar section, the difference current is no longer zero and the

protection operates.

Figure 446: Differential protection on busbar with bus coupler (Single-phase

representation)

Example calculations for busbar high-impedance differential

The protected object in the example for busbar differential protection is a single-

bus system with two zones of protection.

1MRS759142 F

Technical Manual