Functions
2.28 90-%-Stator Earth Fault Protection (ANSI 59N, 64G, 67G)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
205
Earth Current Direction 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 crite-
rion, and the displacement voltage as a necessary enabling condition.
The earth fault current can be measured using a toroidal current transformer or a set of CTs in Holmgreen con-
nection. During a network earth fault, the machine supplies only a negligible earth fault current across the mea-
surement 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 accord-
ing 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 indepen-
dent of the switching status of the network. The earth fault current produced by the loading resistor must always
flow across the measurement location.
Figure 2-86 Earth Fault Direction Detection with Busbar Connection
Consequently, the loading resistor must be situated on the other side of the measurement location (current
transformer, toroidal current transformer) when viewed from the machine. The earthing transformer is prefera-
bly connected to the busbar. Apart from the magnitude of the earth fault current, knowledge of the direction of
this current in relation to the displacement voltage is necessary for the secure detection of a machine earth fault
with busbar connection. The directional border between „machine direction“ and "network direction" can be
altered in the 7UM62 (see the following figure).
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