Siemens siprotec 7SA6, Manual

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Functions
6-31
7SA6 Manual
C53000-G1176-C156-2
Figure 6-17 Logic of the earth fault recognition
Earth Fault
Recognition during
Single-Pole Open
Condition The earth fault recognition is modified during the single-pole open condition with
single-pole automatic reclosure in an earthed system (Figure 6-18). In this case, the
magnitudes of the currents and voltages are monitored in addition to the angles be-
tween the currents.
Figure 6-18 Earth fault recognition during single-pole open condition
Earth Fault Detec-
tion
for Non-Earthed
Systems In non-earthed systems (isolated system star point or resonant-earthed by means of
a Peterson coil) the measured displacement voltage is not used for fault detection.
Furthermore, in these systems a simple earth fault is assumed initially in case of a sin-
gle-phase fault and the fault detection is suppressed in order to avoid an erroneous
pickup as a result of the earth fault initiation transients. After a time delay
T3I0
1PHAS
, which can be set, the fault detection is released again; this is necessary for
the distance protection to still be able to detect a double earth fault with one base point
on a dead-end feeder.
If, however, an earth fault is already present in the system, it is detected by the dis-
placement voltage detection (
3U0> COMP/ISOL.
). In this case, there is no delay
time: an earth fault now occurring in a different phase can only be due to a double
earth fault. If, apart from the displacement measurement (
3U0> COMP/ISOL.
), there
is a fault detection in more than one phase, this is also rated as a double earth fault.
In this way, double earth faults can be detected even if no or only little earth current
flows via the measuring point.
3 I
0
3 I
2
3 I
2
≥
1
3U
0
3I
0
I
Ph
3I
0
3V0>
I
Ph
1203 3I0>
1204 3U0>
earth fault
&
&
≥
1
max(I
Lx
, I
Ly
) <
2 ·min(I
Lx
, I
Ly
)
max(U
Lx
, U
Ly
) <
1.5 ·min(U
Lx
, U
Ly
)
Angle criteria
3I
0
>
1203 3I0>
&
3I
0
I
Lx
earth fault
I
Ly
U
Lx–E
U
Ly–E
for load condition