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0 deg
180 deg
90 deg
270 deg
120 deg
IMinNegSeq
If one or the
other of
currents is too
low, then no
measurement
is done, and
120 degrees
is mapped
External
fault
region
Internal
fault
region
Internal/external
fault boundary
NegSeqROA
(Relay
Operate
Angle)
en05000188-3-en.vsd
IEC05000188 V3 EN-US
Figure 56: Negative sequence current function Relay Operate Angle (ROA)
IminNegSeq
M12541-223 v4
IminNegSeq is set as a multiple of IBase. The local, and the sum of all remote negative
sequence currents are compared separately if they are above the set threshold value
IminNegSeq. If either is below the threshold, no comparison is made. Neither internal nor
external fault is declared in this case. The default value 0.04 ×
IBase can be used if no special
considerations, such as extremely week sources, are taken into account. Observe that
internally, whenever the bias current is higher than 1.5 times
IBase, the actual threshold is
equal to the sum of
Imin 0.1 Ibias. This is in order to prevent wrong decisions of the
internal/external fault discriminator under heavy three-phase external fault conditions with
severe CT saturation.
6.3.3.5
Power transformer in the protected zone
M12541-6 v5
One three-winding transformer or two two-winding transformers can be included in the line
protection zone. The parameters below are used for this purpose. The alternative with one
two-winding transformer in the protected zone is shown in figure
.
RED
670
RED
670
Protected zone
en05000442.vsd
IEC05000442 V1 EN-US
Figure 57: One two–winding transformer in the protected zone
1MRK 505 343-UEN B
Section 6
Differential protection
129
Application manual
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