Earth Current Differential Protection (Ansi 87Gn, Tn); Functional Description - Siemens Siprotec 7UM62 Manual

Functions
2.13

Earth Current Differential Protection (ANSI 87GN, TN)

General

2.13.1 Functional Description

Measuring
Principle
98
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.
Connection scheme 1
Protected
object
Figure 2-45 Connection Schemes of the Earth Current Differential Protection
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.
Connection scheme 2
Protected
object
7UM62 Manual
C53000-G1176-C149-3