ABB REG316 4 Series Operating Instructions Manual page 77

REG 316*4 1MRB520049-Uen / Rev. F
A further difference lies in the fact that the "a" factors only effect
the differential protection, whilst changing the reference values
of the A/D channels effects the currents for the entire protection
system (all functions and measured variables).
The windings of a three-winding transformer normally have dif-
ferent power ratings and a reference power has to be chosen,
which is used for all three windings. All the settings of the pro-
tection are then referred to the reference currents calculated
from the reference power.
250/5 A
25 MVA
1
3
2
600/5 A
20 MVA
Assuming the reference power S
currents I and the "a" factors become:
B
S
B
=
I
B 1
´
3 U 3
TN 1
S
B
=
I
B 2
´
3 U
TN 2
S
B
=
I
B 3
´
3 U 3
TN 3
110 kV
500/5 A
5 MVA 6,3 kV
20 kV
to be 25 MVA, the reference
B
25
=
131 A 1 a
´
110
25
=
722 A a
´
3 20
25
=
2291 A a
´
6 3 .
ABB Switzerland Ltd
131 A
458 A
577 A
HEST 905 004b C
I 250
CT 1
= = =
I 131
B 1
I 600
CT 2
= = =
2
I 722
B 2
I 500
CT 3
= = =
3
I 2291
B 3
. 1 91
. 0 83
. 0 22
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