ABB REG316 4 Series Operating Instructions Manual page 76

ABB Switzerland Ltd
Amplitude compensation factors a1, a2, a3
Factors a1, a2 and a3 facilitate compensating differences be-
tween the rated currents of the protected unit and the c.t's.
The "a" factors are defined by the ratio of the c.t. rated current to
the reference current.
In the case of a two-winding transformer, both windings have the
same rated power and the rated current of the transformer is
taken as the reference current. Providing the factor "a" is cor-
rectly set, all the settings of g, v, b, g-High and I-Inst. are re-
ferred to the rated current of the transformer and not to the rated
primary current of the c.t.
250/5 A
1
25 MVA
2
1000/5 A
I = I = 131 A
B1 TN1
I = I = 722 A
B2 TN2
The reference current is only chosen to be different from the
transformer rated current, if this should be necessary because of
the setting range of factors a1 and a2.
Differences between the rated currents of the c.t's and a two-
winding transformer may also be compensated by adjusting the
reference values of the A/D channels. In this case and assuming
the power ratings of the two windings to be the same, the factors
are set to a1 = a2 = 1. The reference values in the case of the
above example are:
I 131
TN 1
= =
0 524 .
I 250
CT 1
3-38
REG 316*4 1MRB520049-Uen / Rev. F
110 kV 131 A
20 kV 722 A
HEST 905 004a C
I
250
CT 1
= = = .
a 1 191
I 131
TN 1
I 1000
= CT 2 = = .
a 1
I 722
TN 2
I 722
TN 2
= =
I 1000
CT 2
138
0 722 .