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 Figure 2-44) has the largest
number of turns and thus the weighting factor W = 3.
If the higher earth current sensitivity is not necessary, connection according to Figure 2-46 can be used. This is
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 Table 2-6
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 Summation transformer connection L1-L2-L3 with decreased earth fault sensitivity
[diff-ssmischwandler-l1l2l3-stroeme-020926-rei, 1, en_GB]
Figure 2-47 Summation of the currents in the summation transformer on connection L1-L2-L3
Table 2-6 Fault conditions and weighting factors for the CT connection L1-L2-L3
Fehler W W/√3
Ι
1
for Ι
M
= 100 mA
L1-L2-L3 (sym.)
L1-L2
L2-L3
L3-L1
L1-E
L2-E
L3-E
√3
1
2
1
2
1
3
1.00
0.58
1.15
0.58
1.15
0.58
1.73
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
Comparison with the values in the Table 2-5 for L1-L3-E shows that under earth fault conditions the weighting
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.
Functions
2.2 Differential Protection
114 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
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