ABB REO 517 Applications Manual page 27

Requirements
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A CT according to ANSI/IEEE is specified in a little different way. For example a CT
of class C has a specified secondary terminal voltage U
ized value of U (e.g. for a C400 the U
ANSI
secondary e.m.f. E
alANSI
as follows:
⋅ ⋅
E 20 I R
= +
alANSI sn CT
where
Z The impedance (i.e. complex quantity) of the standard ANSI burden for the specific C
bANSI
class (Ω)
U The secondary terminal voltage for the specific C class (V)
ANSI
The CT requirements are fulfilled if:
>
E maximum of E
alANSI alreq
Often an ANSI/IEEE CT also has a specified knee-point voltage U
graphically defined from the excitation curve. The knee-point according to ANSI/IEEE
has normally a lower value than the knee-point according to BS. The rated equivalent
limiting secondary e.m.f. E
estimated to:
≈ ⋅
E 1.3 U
alANSI kneeANSI
The current transformers must have a knee-point voltage U
lowing:
⋅ >
1.3 U maximum of E
kneeANSI
is 400 V). The rated equivalent limiting
ANSI
for a CT specified according to ANSI/IEEE can be estimated
⋅ ⋅
U 20 I R 20 I
= +
ANSI sn CT
for a CT specified according to ANSI/IEEE can be
alANSI
alreq
. There is a few standard-
ANSI
⋅ ⋅
Z
sn bANSI
kneeANSI
that fulfills the fol-
kneeANSI
&KDSWHU 
*HQHUDO
(Equation 7)
(Equation 8)
. This is
(Equation 9)
17