REG 316*4 1MRB520049-Uen / Rev. F ABB Switzerland Ltd
3-39
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.
500/5 A
5 MVA
20 MVA
6,3 kV
600/5 A
458 A
577 A
1
2
3
250/5 A
25 MVA 110 kV 131 A
20 kV
HEST 905 004b C
Assuming the reference power S
B
to be 25 MVA, the reference
currents I
B
and the “a” factors become:
91.1
131
250
I
I
1aA131
1103
25
U3
S
I
1B
1CT
1TN
B
1B
====
´
=
´
83.0
722
600
I
I
2aA722
203
25
U3
S
I
2B
2CT
2TN
B
2B
====
´
=
´
22.0
2291
500
I
I
3aA2291
3.63
25
U3
S
I
3B
3CT
3TN
B
3B
====
´
=
´
To Next Page
Loading...
