2 Functions
64
7SA522 Manual
C53000-G1176-C155-3
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 2-9 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 will take place
during single-pole open condition in earthed power systems (Figure 2-10). In this
case, the magnitudes of the currents and voltages are monitored in addition to the
angles between the currents.
Figure 2-10 Earth fault detection during single-pole open condition
Logical Combina-
tion for Non-
earthed Systems
In non-earthed systems (isolated system star point or resonant-earthed by means of
a Peterson coil) the measured displacement voltage is not used for fault detection.
Furthermore, 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 7,3+$6
which can be set, the fault detection is released again; this is necessary for the dis-
tance protection 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 (8!&203,62/). In this case, there is no delay
time: an earth fault occurring now in a different phase can only be a double earth fault.
If, apart from the displacement measurement (8!&203,62/), there is a fault
detection in more than one phase, this is also rated as a double earth fault. In this way,
double earth faults can be detected even if no or only little earth current flows via the
measuring point.
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