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2.5.2
Functional Description
Measuring
During healthy operation, no starpoint current ISP flows through the starpoint lead.
principle
The sum of the phase currents 3I
When an earth fault occurs in the protected zone, a starpoint current I
pending on the earthing conditions of the power system a further earth current may be
recognized in the residual current path of the phase current transformers (dashed
arrow in Figure 2-27), which is, however, more or less in phase with the starpoint cur-
rent. All currents which flow into the protected zone are defined positive.
Figure 2-27
When an earth fault occurs outside the protected zone (Figure 2-28), a starpoint
current I
current transformers. Since the current direction is normally defined as positive in the
direction of the protected object, this current is in phase opposition with I
Figure 2-28
When a fault without earth connection occurs outside the protected zone, a residual
current may occur in the residual current path of the phase current transformers which
is caused by different saturation of the phase current transformers under strong
through-current conditions. This current could simulate a fault in the protected zone.
Measures must be taken to prevent this current from causing a trip. For this, the re-
stricted earth fault protection provides stabilization methods which differ strongly from
the usual stabilization methods of differential protection schemes since it uses,
besides the magnitude of the measured currents, the phase relationship, too.
Evaluation of Mea-
The restricted earth fault protection compares the fundamental wave of the current
surement
flowing in the starpoint connection, which is designated as 3I
Quantities
the fundamental wave of the sum of the phase currents, which should be designated
in the following as 3I
7SD610 Manual
C53000-G1176-C145-4
Example for an earth fault in a transformer with current distribution
will flow equally; but an equal current 3 I
SP
Example for an earth fault outside a transformer with current distribution
". Thus, the following applies (Figure 2-29):
0
2.5 Restricted Earth Fault Protection (optional)
+ I
+ I
=I
is also approximately zero.
0
L1
L2
L3
must flow through the phase
0
will flow; de-
SP
.
SP
' in the following, with
0
81
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