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Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
117
Figure 2-37 Logic of the earth fault detection
Earth fault detection during single-pole open condition
In order to prevent undesired pickup of the earth fault detection caused by load currents during single-pole open
condition, a modified earth fault detection is used during single-pole open condition in earthed power systems
(Figure 2-38). In this case, the magnitudes of the currents and voltages are monitored in addition to the angles
between the currents.
Figure 2-38 Earth fault detection during single-pole open condition (example: 1-pole dead time L1)
Logical Combination for Non-earthed Systems
In compensated or isolated networks, an earth pickup is only initiated after a pickup of the zero-sequence
current criterion. It should be considered that the zero-sequence voltage criterion with the parameter
1505
3U0> COMP/ISOL.
is used for the confirmation of an earth pickup in case of double earth faults with current
transformer saturation.
The 3I0 threshold is reduced in case of asymmetrical phase-to-phase voltages in order to allow earth pickup
even in the case of double earth faults with a very low zero-sequence current. The zero-sequence voltage cri-
terion is not used solely as the distance measurement for phase-to-earth loops tends to overreach if the earth
current is missing. If the current transformer is saturated and the parameter
1505
is not set to
∞
, an earth fault
detection by means of the I0 criterion alone is not possible and a verification of the pickup by means of the U0
criterion is initiated.