Elimination Of Zero Sequence Currents - ABB RELION 650 Series Applications Manual

Section 6
Differential protection
operate current
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IEC05000187 V2 EN-US
Figure 27: Representation of the restrained, and the unrestrained operate
=
slope
EQUATION1246 V1 EN-US
and where the restrained characteristic is defined by the settings:
1.
2.
3.
4.
5.
6.1.2.2

Elimination of zero sequence currents

A differential protection may operate undesirably due to external earth-faults in cases where
the zero sequence current can flow on only one side of the power transformer, but not on the
other side. This is the case when zero sequence current cannot be properly transformed to the
other side of the power transformer. Power transformer connection groups of the Yd or Dy
type cannot transform zero sequence current. If a delta winding of a power transformer is
earthed via an earthing transformer inside the zone protected by the differential protection
there will be an unwanted differential current in case of an external earth-fault. The same is
true for an earthed star winding. Even if both the star and delta winding are earthed, the zero
sequence current is usually limited by the earthing transformer on the delta side of the power
66
5
4
3
2
Section 1
1
IdMin
0
0
EndSection1
characteristics
D
Ioperate
×
100%
D
Irestrain
IdMin
EndSection1
EndSection2
SlopeSection2
SlopeSection3
Operate
unconditionally
UnrestrainedLimit
Operate
conditionally
Section 2
SlopeSection2
1 2 3
EndSection2
1MRK 504 158-UEN A
Section 3
SlopeSection3
Restrain
4 5
restrain current
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en05000187-2.vsd
Application manual
(Equation 14)
M15266-286 v8