Siemens siprotec 7SA6, Manual

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Functions
6-45
7SA6 Manual
C53000-G1176-C156-2
Double Earth-
Faults in Earthed
Systems In systems with earthed starpoint (effective or low-resistant), each contact of a phase
with earth results in a short-circuit condition which must be isolated immediately by the
closest protection systems. Fault detection occurs in the faulted loop associated with
the faulted phase.
With double earth faults, fault detection is generally in two phase-earth loops. If both
earth loops are in the same direction, a phase-phase loop may also pick-up. It is pos-
sible to restrict the fault detection to particular loops in this case. It is often desirable
to block the phase-earth loop of the leading phase, as this loop tends to overreach
when there is infeed from both ends to a fault with a common earth fault resistance
(address
1221 2Ph-E faults
=
Block leading Ø
). Alternatively, it is also possi-
ble to block the lagging phase-earth loop (address
2Ph-E faults
=
Block lag-
ging Ø
). All the affected loops can also be evaluated (address
2Ph-E faults
=
All
loops
), or only the phase-phase loop (address
1221 2Ph-E faults
=
Ø-Ø loops
only
) or only the phase-earth loops (address
2Ph-E faults
=
Ø-E loops only
).
A prerequisite for these restrictions is that the relevant loops indicate fault locations
which are close together and within the reach of the first zone Z1. The loops are con-
sidered to be close together when they have the same direction and do not differ by
more than a factor 1,5 (largest to smallest impedance). This is to avoid in case of mul-
tiple faults with separate fault location the closer fault is eliminated from the evaluation
by configured restrictions. Furthermore, the measurement phase-phase can only be
carried out if two earth faults are close to each other in the determined direction.
In Table 6-7 the measured values used for the distance measurement in earthed sys-
tems during double earth faults are shown.
During three phase faults the fault detection of all three phase-phase loops usually oc-
curs. In this case the three phase-phase loops are evaluated. If earth fault detection
also occurs, the phase-earth loops are also evaluated.
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
Evaluated
Loop
Setting
Parameter
1221
L1–E, L2–E, L1–L2
L2–E, L3–E, L2–L3
L1–E, L3–E, L3–L1 L2–E, L1–L2
L3–E, L2–L3
L1–E, L3–L1
2Ph-E faults =
Block leading Ø
L1–E, L2–E, L1–L2
L2–E, L3–E, L2–L3
L1–E, L3–E, L3–L1 L1–E, L1–L2
L2–E, L2–L3
L3–E, L3–L1
2Ph-E faults =
Block lagging Ø
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
2Ph-E faults =
All loops
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