Siemens SIPROTEC 7UT613 series Manual page 128

2 Functions
128
Figure 2-46 Summation transformer connection L1-L2-L3 with decreased earth fault sensi-
tivity
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
Fault
L1-L2-L3 (sym.)
L1-L2
L2-L3
L3-L1
L1-E
L2-E
L3-E
Comparison with the values in the table 2-5 or 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)
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.
The type 4AM5120 is recommended for summation current transformers. These
transformers have different input windings which allow for summation of the currents
with the ratio 2 : 1 : 3 as well as matching of different primary currents of the main CTs
to a certain extent. Figure 2-48 shows the winding arrangement.
The nominal input current of each summation CT must match the nominal secondary
current of the connected main CT set. The output current of the summation CT (= input
W W/√3
√3
1,00
1 0,58
2 1,15
1 0,58
2 1,15
1 0,58
3 1,73
2
.
I for I
= 100 mA
1 M
1.00 ·I
N
1.73 ·I
N
0.87 I
N
1.73 ·I
N
0.87 ·I
N
1.73 ·I
N
0.58 ·I
N
7UT613/63x Manual
C53000-G1176-C160-2