Siemens SIPROTEC 4 7UT6 Series Manual page 114

Functions
2.2 Differential Protection
The table shows that the differential protection is more sensitive to earth faults and to double earth faults
than to those without earth path component. This increased sensitivity is due to the fact that the summation
transformer winding in the CT starpoint connection (IE, residual current (see
number of turns and thus the weighting factor W = 3.
If the higher earth current sensitivity is not necessary, connection according to
reasonable in earthed systems with particularly low zero sequence impedance where earth fault currents may
be larger than those under twophase fault conditions. With this connection, the values given in
below can be recalculated for the seven possible fault conditions in solidly earthed networks.
[diff-ssmischwandler-l1l2l3-020926-rei, 1, en_GB]
Figure 2-46
[diff-ssmischwandler-l1l2l3-stroeme-020926-rei, 1, en_GB]
Figure 2-47
Table 2-6
Fehler
L1-L2-L3 (sym.)
L1-L2
L2-L3
L3-L1
L1-E
L2-E
L3-E
Comparison with the values in the
factor W is less than with the standard connection. Thus the thermal loading is reduced to 36 %, i.e.
(1.73/2,89) 2 .
The described connections are examples. Certain phase preferences (especially in systems with non-earthed
neutral) can be obtained by cyclic or acyclic exchange of the phases. Further increase of the earth current can
be performed by introducing an auto-CT in the residual path, as a further possibility.
114
Summation transformer connection L1-L2-L3 with decreased earth fault sensitivity
Summation of the currents in the summation transformer on connection L1-L2-L3
Fault conditions and weighting factors for the CT connection L1-L2-L3
W
√3
1
2
1
2
1
3
Table 2-5
W/√3
1.00
0.58
1.15
0.58
1.15
0.58
1.73
for L1-L3-E shows that under earth fault conditions the weighting
Figure 2-44) has the largest
Figure 2-46 can be used. This is
Table 2-6
for Ι = 100 mA
Ι
1 M
1.00 ·Ι
N
1.73 ·Ι
N
0.87 Ι
N
1.73 ·Ι
N
0.87 ·Ι
N
1.73 ·Ι
N
0.58 ·Ι
N
SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016