Functions
2.8 Earth Fault Protection in Earthed Systems (optional)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
225
Figure 2-108 Zero-sequence power protection
Figure 2-108 shows the logic diagram. The tripping time depends on the level of the compensated zero-se-
quence power S
r
as defined above. For meshed earthed systems the zero-sequence voltage and the zero-se-
quence current increase towards the earth fault location. The inverse characteristic results in the shortest
command time for the relay closest to the fault. The other relays then reset.
Phase current stabilization
Asymmetrical load conditions in multiple-earthed systems or different current transformer errors can result in a
zero-sequence current. This zero-sequence current could cause faulty pickup of the earth current stages if low
pickup thresholds are set. To avoid this, the earth current stages are stabilized by the phase current: as the
phase currents increase, the pickup thresholds are increased (Figure 2-109). The stabilization factor (= slope)
can be changed with parameter Iph-STAB. Slope (address 3104). It applies to all stages.
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