Siemens siprotec 7SA6, Manual

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Functions
6-50
7SA6 Manual
C53000-G1176-C156-2
The 7SA6 enables the user to detect all foot points of a multiple earth fault.
PHASE
PREF.2phe
=
All loops
means that each earth fault point on a protected line is
switched off independent of the preference. It can also be combined with a different
preference. For a transformer feeder, for example, any foot point can be switched off
following occurrence of a double earth fault, whereas
L1 (L3) acyclic
is consist-
ently valid for the remainder of the system.
If the earth fault detection threatens to pick up due to fault inception transients follow-
ing the occurrence of a single earth fault, the detection can be delayed with setting ad-
dress
1206 T3I0 1PHAS.
Usually the presetting (0.04 s) is sufficient. For large res-
onant-earthed systems the time delay should be increased. Set parameter
T3I0
1PHAS
to
∞
if the earth current threshold can also be exceeded during steady-state
conditions. Then, even with high earth current, no single-phase pick up is possible an-
ymore. Double earth faults are, however, detected correctly and evaluated according
to the preference mode (Section 6.2.3.1, paragraph “Double Earth Faults in Non-
earthed Systems”).
If a double earth fault occurs right after a single earth fault, it is detected and evaluated
according to the preference scheme. The already existing earth fault is detected by
the zero-sequence voltage (address
1205 3U0> COMP/ISOL.
). Please note that
three times the zero-sequence voltage 3U
0
is relevant. With a full displacement its val-
ue will be
√
3-times the phase-to-phase voltage. After detection of 3U
0
, the time delay
T3I0 1PHAS
is not active anymore: an earth fault ocurring then in a different phase
can only be a double earth fault.
Line Energization
onto a Dead Fault
To determine the reaction of the distance protection during closure of the circuit break-
er onto a dead fault, the parameter in address
1232 SOTF zone
is used. The setting
Inactive
specifies that there is no special reaction, i.e. all distance stages operate
according to their set zone parameters. The setting
Zone Z1B
causes all faults inside
the overreaching zone Z1B to be cleared without delay following closure of the circuit
breaker. The setting
Pickup
implies that the non-delayed tripping following line ener-
gization is activated for all recognized faults in any zone (i.e. with general fault detec-
tion of the distance protection).
Load Area
(only for
Impedance Pick-up)
On long heavily loaded lines, the risk of encroachment of the load impedance into the
tripping characteristic of the distance protection may exist. To exclude the risk of un-
wanted fault detection by the distance protection during heavy load flow, a load trap-
ezoid characteristic may be set for tripping characteristics with a large R-reach, which
excludes such unwanted fault detection by overload. This load area is considered in
the description of the tripping characteristics (refer to Figure 6-29, Subsubsection
6.2.4.1).
The R–values
R load (Ø-E)
(address
1241
) and
R load (Ø-Ø)
(address
1243
)
must be set somewhat (approx. 10 %) smaller than the minimum load impedance
which may occur. The minimum load impedance results when the maximum load cur-
rent and minimum operating voltage exist.
Calculation example:
110 kV overhead line 150 mm
2
with the following data:
maximum transferrable load
P
max
= 100 MVA corresponding to
I
max
= 525 A
minimum operating voltage
U
min
= 0.9 U
N