Siemens siprotec 7SA6 User Manual page 227

Double Earth-
Faults in
Non-earthed
Systems
7SA6 Manual
C53000-G1176-C133-1
Table 6-7 Evaluation of the measured loops for double loop faults in an earthed system in
case both earth faults are close to each other
Fault detection
Loop
L1–E, L2–E, L1–L2
L2–E, L3–E, L2–L3
L1–E, L3–E, L3–L1
L1–E, L2–E, L1–L2
L2–E, L3–E, L2–L3
L1–E, L3–E, L3–L1
L1–E, L2–E, L1–L2
L2–E, L3–E, L2–L3
L1–E, L3–E, L3–L1
L1–E, L2–E, L1–L2
L2–E, L3–E, L2–L3
L1–E, L3–E, L3–L1
L1–E, L2–E, L1–L2
L2–E, L3–E, L2–L3
L1–E, L3–E, L3–L1
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.
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
result 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.
U
L1
E
U
L3
a) Healthy System, without Earth Fault
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.
Evaluated
Loop
L2–E, L1–L2
L3–E, L2–L3
L1–E, L3–L1
L1–E, L1–L2
L2–E, L2–L3
L3–E, L3–L1
L1–E, L2–E, L1–L2
L2–E, L3–E, L2–L3
L1–E, L3–E, L3–L1
L1–L2
L2–L3
L3–L1
L1–E, L2–E
L2–E, L3–E
L1–E, L3–E
U
L2
Functions
Setting
Parameter 1221
3K( IDXOWV
%ORFN OHDGLQJ ‘
3K( IDXOWV
%ORFN ODJJLQJ ‘
3K( IDXOWV
$OO ORRSV
3K( IDXOWV
‘‘ ORRSV RQO\
3K( IDXOWV
‘( ORRSV RQO\
U = 0
E
L1
U U
L3 L2
b) Earth Fault in Phase L1
6-45