2.3 Restricted Earth Fault Protection
141
7UT613/63x Manual
C53000-G1176-C160-2
Figure 2-56 Restricted earth fault protection on a generator or motor with earthed starpoint
The restricted earth fault protection can operate on one of the sides of the main pro-
tected object (power transformer, generator, motor, reactor) or on a further protected
object, according to the topology configured. In case of auto-transformers, it is as-
signed to the auto-windings. Furthermore, it is presumed that the assignment of the
different measuring locations to the sides of the main protected object or to a further
protected object as well as the assignment of the 1-phase current input for the star-
point current has been performed correctly according to the Subsection „Topology of
the Protected Object“.
The 7UT613/63x is equipped with two such protective functions that can be used in-
dependent of each other and at various locations. You can, for example, implement
an earth fault differential protection for both of the windings at a YNyn transformer that
is earthed at both starpoints. Or use the first earth fault differential protection for an
earthed transformer winding and the second for a further protective object, e.g. a
neutral reactor. Allocation of both earth fault differential protection functions to the
sides or measuring locations are done according to Section „Assignment of protection
functions to measuring locations.sides“are performed.
2.3.2 Function Description
Measuring Princi-
ple
During healthy operation, no starpoint current I
Ctrl
flows through the starpoint lead. The
sum of the phase currents 3I
0
=I
L1
+ I
L2
+ I
L3
is almost zero.
When an earth fault occurs in the protected zone, a starpoint current I
Ctrl
will flow; de-
pending on the earthing conditions of the power system a further earth current may be
recognised in the residual current path of the phase current transformers (dashed
arrow in Figure 2-57), which is, however, more or less in phase with the starpoint cur-
rent. All currents which flow into the protected zone are defined positive.
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