Siemens siprotec 7SA6, Manual

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Functions
6-46
7SA6 Manual
C53000-G1176-C156-2
Double Earth-
Faults in
Non-earthed Sys-
tems 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 6-26).
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 re-
sult in an impedance value that is equal to zero. Therefore a single-phase pick-up
phase-earth without earth current pickup is avoided in the 7SA6.
Figure 6-26 Earth fault in non-earthed systems
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
pick-up. In case of an overcurrent pick-up there may also be a phase current pick-up.
There are special measures against such undesirable pick-ups (see Subsection
6.2.1).
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 7SA6 the following double earth fault preferences
PHASE PREF.2phe
can be
selected:
•
acyclic L3 before L1 before L2, short form:
L3 (L1) acyclic
;
•
acyclic L1 before L3 before L2, short form:
L1 (L3) acyclic
;
•
acyclic L2 before L1 before L3, short form:
L2 (L1) acyclic
;
•
acyclic L1 before L2 before L3, short form:
L1 (L2) acyclic
;
•
acyclic L3 before L2 before L1, short form:
L3 (L2) acyclic
;
•
acyclic L2 before L3 before L1, short form:
L2 (L3) acyclic
;
•
cyclic L3 before L1 before L2 before L3, short form:
L3 (L1) cyclic
;
•
cyclic L1 before L3 before L2 before L1, short form:
L1 (L3) cyclic
;
•
all loops are measured (no preference)
All loops
.
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 ordering option “Earth Fault Detection in Non-Earthed Systems”
(see Section 6.13).
The 7SA6 also enables the user to switch off both fault locations of a double earth
fault. Set the double earth fault preference to
All loops
.
E
U
L1
U
L2
U
L3
E
U
L1
= 0
U
L2
U
L3
a) Healthy System, without Earth Fault b) Earth Fault in Phase L1