Different Primary Nominal Currents; Two-Phase Connection; Figure 5.6 Matching Unequal Primary Currents; Figure 5.7 Two-Phase Connection L1 - L3 - Siemens SIPROTEC 7SD600 System Manual

5
Preparation instructions
Numerical line differential protection SIPROTEC 7SD600 V3.0
5.2.4.4

Different primary nominal currents

If the current transformers of the two line ends have
different primary nominal currents, the currents need
to be matched via the matching transformer. Mat
ching is carried out at the line end with the lower pri
mary nominal current.
As current inputs are galvanically isolated in the
4AM4930, the matching transformers can be inserted
in an auto-transformer mode. This makes better use
of the rating of the matching transformer and enables
implementation of a lower-rating type, such as the
4AM51 70-7AA. Figure 5.6 shows a sample connec
tion.
The actual transformation ratio of the matching trans
former is determined as follows:
I
N primary
r minimum + minimum
I
I
N primary
This means that the turn ratio is
I
N
N primary
1
minimum + minimum
N I
2 N primary
with
N - relative number of turns of the side facing the
1
main current transformer,
N - relative number of turns of the side facing the
2
summation transformer.
Example:
Current transformer at line end I:
Current transformer at line end II:
Matching is carried out at line end II:
I for lower rated current
N
N primary
1
+
N
I for higher rated current
2
N primary
N
+ 400 A
1
+ 0, 8 corresponds to e.g. 40 50 turns
N 500 A
2
If the earth-fault sensitivity is to be increased in ac
cordance with section 5.2.4.3, the matching transfor
mer inserted in the earth current path, as shown in
Figure 5.6, can also be used for this purpose.
5 - 8
for higher rated current
for lower rated current
for lower rated current
for higher rated current
500 A/1 A
400 A/1 A
System manual Order no. E50417-G1176-C069-A1
MT
N
1
N
N
1
N
N
1
N
L1 L2 L3

Figure 5.6 Matching unequal primary currents

5.2.4.5

Two-phase connection

If a system without an earthed neutral only has two
current transformers, the phase on the device wi
thout a current transformer remains free (Figure 5.7).
In the case of a double earth fault, the fault is not de
tected by the protection. This must conform to the
double earth fault preference in the whole of the elec
trically connected system. Please note that the same
phase must also be free at the other line end, even if
it is fitted with three current transformers.
L1 L2 L3

Figure 5.7 Two-phase connection L1 - L3

MT
C Y
2
2
H
A
2
1
F
L
3
M Z
Pluggable links D-G
2
B-E
MT
C Y
2
H
A
1
F
L
3
M Z
Pluggable links D-G
B-E
Siemens AG January 1999