2.21 90-%-Stator Earth Fault Protection (ANSI 59N, 64G, 67G)
129
7UM61 Manual
C53000-G1176-C127-3
Figure 2-41
Unit Connection with Earthing Transformer
Figure 2-42
Unit Connection with Earthing Transformer
Earth Current Di-
rection Detection
For machines in busbar connection, it is not possible to differentiate between network
earth faults and machine earth faults using the displacement voltage alone. In this
case the earth fault current is used as a further criterion, and the displacement voltage
as a necessary enabling condition.
This can be measured using a toroidal current transformer or a set of CTs in
Holmgreen connection. During a network earth fault, the machine supplies only a neg-
ligible earth fault current across the measurement location, which must be situated
between the machine and the network. During a machine earth fault, the earth fault
current of the network is available. However, since the network conditions generally
vary according to the switching status of the network, a loading resistor, which supplies
an increased earth fault current on the occurrence of a displacement voltage, is used
in order to obtain definite measurement conditions independent of the switching status
of the network. The earth fault current produced by the loading resistor must always
flow across the measurement location.
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