2 Functions
112
7SD5 Manual
C53000-G1176-C169-1
The earth fault recognition alone does not cause a general pickup of the distance pro-
tection, but merely controls the further fault detection modules. It is only alarmed in
case of a general fault detection.
Figure 2-29 Logic of the earth fault detection
Earth Fault Recog-
nition during
Single-Pole Open
Condition
In order to prevent undesired pickup of the earth fault detection, caused by load cur-
rents during single-pole open condition, a modified earth fault detection is used during
single-pole open condition in earthed power systems (Figure 2-30). In this case, the
magnitudes of the currents and voltages are monitored in addition to the angles
between the currents.
Figure 2-30 Earth fault recognition during single-pole open condition
Logical Combina-
tion for Non-
earthed Systems
In non-earthed systems (isolated system starpoint or resonant-earthed by means of a
Peterson coil) the measured displacement voltage is not used for fault detection. Fur-
thermore, in these systems a simple earth fault is assumed initially in case of a single-
phase fault and the fault detection is suppressed in order to avoid an erroneous pickup
as a result of the earth fault initiation transients. After a time delay T3I0 1PHAS which
can be set, the fault detection is released again; this is necessary for the distance pro-
tection to still be able to detect a double earth fault with one base point on a dead-end
feeder.
If, however, an earth fault is already present in the system, it is detected by the dis-
placement voltage detection (3U0> COMP/ISOL.). In this case, there is no delay
time: an earth fault occurring now in a different phase can only be a double earth fault.
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