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Functions
2.15 Earth Current Differential Protection (ANSI 87GN,TN)
2.15
Earth Current Differential Protection (ANSI 87GN,TN)
The earth current differential protection detects earth faults in generators and transformers with a low-ohmic or
solid starpoint earthing. It is selective, and more sensitive than the classical differential protection (see Section
2.14.1).
A typical application of this protection function are configurations where multiple generators are connected to
one busbar and one generator has a low-ohmic earthing. Another application would be transformer windings
in wye connection.
For applications such as auto-transformers, starpoint earthing transformers and shunt reactors, Siemens rec-
ommends that the 7UT612 protective device be used instead.
For high-ohmic earthing of generators, the earth fault protection function (Section 2.28) is used.
2.15.1
Functional Description
Connection Variants
The following figure shows two typical implementations. In connection scheme 1, the zero sequence current is
calculated from the measured phase currents, and the starpoint current is measured directly. This application
is the version for transformers and for the generator with direct (low-ohmic) earthing.
In connection scheme 2, both zero sequence currents are calculated from the measured phase currents. The
protected object is located between the current transformers. This measuring method should be used for gen-
erators in busbar connection, where multiple generators feed the busbar and any one of the generators is
earthed.
Figure 2-47
128
Connection schemes of the earth current differential protection
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
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