ABB RED650, Applications Manual

Page: 84 / 454

With 1½ breaker configurations, normally the line protection is fed from two CTs.
Avoiding to add the currents from the two CTs externally before entering the IED
is important as this will enable possible bias current from both CTs to be
considered in the current differential algorithm, and in that way assuring that the
correct restrain will be possible, as shown in figure
29 .
IEC05000428-2-en.vsdx
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Protected zone
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Figure 29: Line protection with 1½ breaker configurations, fed from two CTs
6.1.2.1 Power transformers in the protected zone
M12022-47 v6
Line differential protection can also be applied with in-line power transformers in
the protected zone. Such an in-line power transformer can be situated in tap, as
shown in Figure
30 or in one end of a two-terminal line. Observe that the currents
of the in-line power transformer are measured by the IED on the low voltage side.
A one three-winding transformer can be included.
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Figure 30: Transformer situated in tap
A current differential protection including power transformers must be
compensated for transformer turns ratio and phase shift/vector group. In the line
differential function this compensation is made in the software algorithm, which
Section 6
1MRK 505 393-UEN B
Differential protection
78 Line differential protection RED650 2.2 IEC
Application manual