Current Transformers According To Ansi/Ieee - ABB REC670 Applications Manual

Section 24
Requirements
24.1.7.3
636

Current transformers according to ANSI/IEEE

Current transformers according to ANSI/IEEE are partly specified in different ways. A
rated secondary terminal voltage V
secondary terminal voltage the CT will deliver to a standard burden at 20 times rated
secondary current without exceeding 10 % ratio correction. There are a number of
standardized V values for example, V
ANSI
corresponding rated equivalent limiting secondary e.m.f. E
follows:
× × +
= 20 I R V
E
S N CT
a lANS I
EQUATION1682 V1 EN-US
where:
Z The impedance (that is, with a complex quantity) of the standard ANSI burden for the specific C
bANSI
class (W)
V The secondary terminal voltage for the specific C class (V)
ANSI
The CTs according to class C must have a calculated rated equivalent limiting
secondary e.m.f. E
alANSI
>
E maximum of E
alANSI
EQUATION1384 V2 EN-US
A CT according to ANSI/IEEE is also specified by the knee point voltage V
that is graphically defined from an excitation curve. The knee point voltage V
normally has a lower value than the knee-point e.m.f. according to IEC and BS.
V can approximately be estimated to 75 % of the corresponding E
kneeANSI
to IEC 61869-2. Therefore, the CTs according to ANSI/IEEE must have a knee point
voltage V that fulfills the following:
kneeANSI
> ×
V 0.75 (max imum of E
kneeANSI
EQUATION2101 V2 EN-US
The following guide may also be referred for some more application aspects of ANSI
class CTs: IEEE C37.110 (2007), IEEE Guide for the Application of Current
Transformers Used for Protective Relaying Purposes.
is specified for a CT of class C. V
ANSI
is 400 V for a C400 CT. A
ANSI
= × × +
20 I R 20 I
ANS I S N CT
that fulfils the following:
alreq
)
alreq
1MRK 511 401-UUS A
ANSI
can be estimated as
alANSI
× ×
Z
S N bANS I
al
Bay control REC670 2.2 ANSI
Application manual
M11623-22 v6
is the
(Equation 135)
(Equation 136)
kneeANSI
kneeANSI
according
(Equation 137)