2.2 Differential Protection
127
7UT613/63x Manual
C53000-G1176-C160-2
Figure 2-45 Summation of the currents in the summation transformer on connection L1-L3-E
For the connection L1-L3-E (see figure 2-44), the weighting factors W of the summa-
tion currents IM for the various fault conditions and the ratios to that given by the three-
phase symmetrical faults are shown in table 2-5. On the right hand side is the comple-
mentary multiple of rated current I
M
= 100 mA which W/√3 would have to be in order
to arrive at the summation current I
1
. If the current setting values are multiplied with
this factor, the actual pickup values result.
Table 2-5 Fault conditions and weighting factors for the CT connection L1-L3-E
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 sensi-
tivity is due to the fact that the summation transformer winding in the CT starpoint con-
nection (I
E
, residual current (refer to 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 se-
quence impedance where earth fault currents may be larger than those under two-
phase 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.
Fault W W/√3 I
1
for I
M
= 100 mA
L1-L2-L3 (sym.)
L1-L2
L2-L3
L3-L1
L1-E
L2-E
L3-E
√3
2
1
1
5
3
4
1,00
1,15
0,58
0,58
2,89
1,73
2,31
1.00 · I
N
0.87 · I
N
1.73 · I
N
1.73 · I
N
0.35 · I
N
0.58 · I
N
0.43 · I
N
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