Functions
98
7UM62 Manual
C53000-G1176-C149-3
2.13
Earth Current Differential Protection (ANSI 87GN, TN)
General
The earth current differential protection detects earth faults in generators and trans-
formers with a low-ohmic or solid starpoint earthing. It is selective, and more sensitive
than the classical differential protection (see Section 2.12).
A typical application of this protection function are configurations where multiple gen-
erators are connected to one busbar and one generator has a low-ohmic earthing. An-
other application would be transformer windings in wye connection.
For applications such as auto-transformers, starpoint earthing transformers and shunt
reactors, Siemens recommends to use the 7UT612 protective relay instead.
For high-ohmic earthing of generators, the earth fault protection function (Section
2.26) is used.
Figure 2-45 shows two typical implementations. In connection scheme 1, the zero se-
quence current is calculated from the measured phase currents, whereas 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 mea-
sured phase currents. The protected object is located between the current transform-
ers. This measuring method should be used for generators in busbar connection,
where multiple generators feed the busbar and any one of the generators is earthed.
Figure 2-45 Connection Schemes of the Earth Current Differential Protection
2.13.1 Functional Description
Measuring
Principle
As can be seen in Figure 2-45, there are 2 possible implementations of the earth fault
differential protection which differ only in their method of determining the zero se-
quence current. This is shown in Figure 2-46, along with the definition of the current
direction. The general definition is: Reference arrows run in positive direction to the
protected object.
Protected
object
Connection scheme 1
Protected
object
Connection scheme 2
