Power Transformers In The Protected Zone - ABB RELION 650 Series Applications Manual

Section 6
Differential protection
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 31.
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Figure 31:
6.2.2.1

Power transformers in the protected zone

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 32
transformer are measured by the IED on the low voltage side. A one three-winding transformer
can be included.
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Figure 32: 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 eliminates the need for auxiliary
interposing current transformers. Necessary parameters, such as transformer rated voltages
and phase shift, must be entered via the Parameter Setting tool or the LHMI.
Another concern with differential protection on power transformers is that a differential
protection may operate unwanted due to external earth faults in cases where the zero-
sequence current can flow only on one side of the power transformer but not on the other
side, as in the case of Yd or Dy phase shift/vector groups. This is the situation when the zero-
sequence current cannot be transformed from one side to the other side of the transformer.
To make the differential protection insensitive to external earth faults in these situations, the
zero-sequence current must be eliminated from the terminal currents so that it does not
appear as a differential current. This was previously achieved by means of intermediate
current transformers. The elimination of zero-sequence current is done numerically and no
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Line protection with 1½ breaker configurations, fed from two CTs
or in one end of a two-terminal line. Observe that the currents of the in-line power
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Protected zone
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1MRK 505 363-UEN A
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M12022-47 v6
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Application manual