Siemens siprotec 7SD5, Руководство пользователя, Страница: 122 / 698

2 Functions
122
7SD5 Manual
C53000-G1176-C169-1
During three phase faults the fault detection of all three phase-phase loops usually
occurs. In this case the three phase-phase loops are evaluated. If earth fault detection
also occurs, the phase-earth loops are also evaluated.
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 2-37).
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 7SD5.
Figure 2-37 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.
7SD5 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 7SD5 the following double earth fault preferences (Parameter
1520 PHASE
PREF.2phe
) can be selected:
L1-E, L2-E, L1-L2
L2-E, L3-E, L2-L3
L1-E, L3-E, L3-L1 L1-L2
L2-L3
L3-L1 2Ph-E faults = Ø-Ø loops only
L1-E, L2-E, L1-L2
L2-E, L3-E, L2-L3
L1-E, L3-E, L3-L1 L1-E, L2-E
L2-E, L3-E
L1-E, L3-E 2Ph-E faults = Ø-E loops only
Loop pickup Evaluated loop(s) Setting of parameter 1521
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
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