2.2 Distance protection
69
7SA522 Manual
C53000-G1176-C155-3
Double Earth Faults
in Non-earthed
Systems
In isolated or resonant-earthed networks a single earth fault does not result in a short
circuit current flow. There is only a displacement of the voltage triangle (Figure 2-15).
For the system operation this state is no immediate danger. The distance protection
must not pick up in this case even though the voltage of the phase with the earth fault
is equal to zero in the whole galvanically connected system. Any load currents will
result in an impedance value that is equal to zero. Therefore a single-phase pickup
phase-earth without earth current pickup is avoided in the 7SA522.
Figure 2-15 Earth fault in non-earthed neutral system
With the occurrence of earth faults — especially in large resonant-earthed systems —
large fault inception transient currents can appear that may evoke the earth current
pickup. In case of an overcurrent pickup there may also be a phase current pickup.
7SA522 provides special measures against such undesirable pickups.
With the occurrence of a double earth fault in isolated or resonant-earthed systems it
is sufficient to switch off one of the faults. The second fault may remain in the system
as a simple earth fault. Which of the faults is switched off depends on the double earth
fault preference which is set the same in the whole galvanically-connected system.
With 7SA522 the following double earth fault preferences (Parameter 3+$6(
35()SKH) can be selected:
In all eight preference options, one earth fault is switched off according to the prefer-
ence scheme. The second fault can remain in the system as a simple earth fault. It can
be detected with the Earth Fault Detection in Non-earthed Systems (optional).
The 7SA522 also enables the user to switch off both fault locations of a double earth
fault. Set the double earth fault preference to $OOORRSV.
Table 2-3 lists all measured values used for the distance measuring in isolated or res-
onant-earthed systems.
acyclic L3 before L1 before L2 L3 (L1) ACYCLIC
acyclic L1 before L3 before L2 L1 (L3) ACYCLIC
acyclic L2 before L1 before L3 L2 (L1) ACYCLIC
acyclic L1 before L2 before L3 L1 (L2) ACYCLIC
acyclic L3 before L2 before L1 L3 (L2) ACYCLIC
acyclic L2 before L3 before L1 L2 (L3) ACYCLIC
cyclic L3 before L1 before L2 before L3 L3 (L1) CYCLIC
cyclic L1 before L3 before L2 before L1 L1 (L3) CYCLIC
all loops are measured All loops
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