systems regardless of the conditioning of the system neutral. It is characterised by an increased sensitivity for
earth faults.
[diff-ssmischwandler-l1l3e-020926-rei, 1, en_GB]
Figure 2-44 Summation Transformer Connection L1-L3-E
For a symmetrical three-phase current (where the earth residual component Ι
E
= 0) the single-phase summa-
tion current is W = 3 times the winding unit value, as shown in Figure 2-45, i.e. the summation flux (ampere
turns) is the same as it would be for single-phase current √3 times the value flowing through the winding with
the least number of turns (ratio 1). For three-phase symmetrical fault currents equal to rated current 1 x Ι
N
,
the secondary single-phase current is Ι
M
= 100 mA. All relay characteristic operating values are based on this
type of fault and this current.
[diff-ssmischwandler-l1l3e-stroeme-020926-rei, 1, en_GB]
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 summation currents Ι
M
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 complementary multiple of rated current Ι
M
= 100 mA which W/√3
would have to be in order to arrive at the summation current Ι
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
Fault 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
2
1
1
5
3
4
1.00
1.15
0.58
0.58
2.89
1.73
2.31
1.00 · Ι
N
0.87 · Ι
N
1.73 · Ι
N
1.73 · Ι
N
0.35 · Ι
N
0.58 · Ι
N
0.43 · Ι
N
Functions
2.2 Differential Protection
SIPROTEC 4, 7UT6x, Manual 113
C53000-G1176-C230-5, Edition 09.2016
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