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4 - 3
Siemens AG ⋅ January 1999
The summation transformer 4AM4930 combines the
secondary currents of the main current transformer
and transforms them to a secondary nominal current
of 20 mA. Figure 4.2 shows the standard connection
of the 4AM4930.
A
B
C
D
E
F
G
H
I
K
L
MZ
U
Y
to 7SD600
L1
L2 L3
The connection shown in Figure 4.2 results in a winĆ
ding factor W for the various types of fault with a raĆ
tio relative to a three-phase symmetrical fault accorĆ
ding to table 4.1. The primary current I
1
required to
produce the 20 mA secondary current which is deriĆ
ved from the reciprocal value of W is also shown.
Multiplying the setting values by these factors will
give you the actual pick-up value.
It is clear from the table that the line differential proĆ
tection 7SD600 is more sensitive to single or double
earth faults than to short-circuits without earth. This
increased sensitivity is because the winding I
E
(see
Figure 4.2), of the summation transformer, located in
the neutral conductor has the largest number of turns
so that the the earth current is input with the winding
factor 3.
Table 4.1 Types of faults and winding factors
for standard connection L1-L3-E
Fault W
W/
3
Ǹ
I
1
for i
M
=20mA
L1-L2-L3 (sym.)
ĂĂĂĂĂ 3
Ǹ
1.00 1.00 x I
N
L1-L2 2 1.15 0.87 x I
N
L2-L3 1 0.58 1.73 x I
N
L3-L1 1 0.58 1.73 x I
N
L1-E 5 2.89 0.35 x I
N
L2-E 3 1.73 0.58 x I
N
L3-E 4 2.31 0.43 x I
N
Due to the difference in the number of primary turns
and the various taps on the winding, different connecĆ
tion are possible. Connections for double earth fault
preference, earth fault sensitivity, transformation ratio
matching, different summation ratios and other main
CT terminations (e.g. two-phase connection may be
selected).
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