ABB Relion REC670, Applications Manual

Page: 642 / 658

24.1.7.3 Current transformers according to ANSI/IEEE
M11623-22 v6
Current transformers according to ANSI/IEEE are partly specified in different ways. A
rated secondary terminal voltage V
ANSI
is specified for a CT of class C. V
ANSI
is the
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
ANSI
values for example, V
ANSI
is 400 V for a C400 CT. A
corresponding rated equivalent limiting secondary e.m.f. E
alANSI
can be estimated as
follows:
a lANS I
S N CT ANS I S N CT S N bANS I
E
20 I R V 20 I R 20 I Z =
× × + = × × + × ×
EQUATION1682 V1 EN-US
(Equation 135)
where:
Z
bANSI
The impedance (that is, with a complex quantity) of the standard ANSI burden for the specific C
class ( W )
V
ANSI
The secondary terminal voltage for the specific C class (V)
The CTs according to class C must have a calculated rated equivalent limiting
secondary e.m.f. E
alANSI
that fulfils the following:
alANSI alreq
E maximum of E
>
EQUATION1384 V2 EN-US
(Equation 136)
A CT according to ANSI/IEEE is also specified by the knee point voltage V
kneeANSI
that is graphically defined from an excitation curve. The knee point voltage V
kneeANSI
normally has a lower value than the knee-point e.m.f. according to IEC and BS.
V
kneeANSI
can approximately be estimated to 75 % of the corresponding E
al
according
to IEC 61869-2. Therefore, the CTs according to ANSI/IEEE must have a knee point
voltage V
kneeANSI
that fulfills the following:
kneeANSI alreq
V 0.75 (max imum of E )
> ×
EQUATION2101 V2 EN-US
(Equation 137)
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.
Section 24
1MRK 511 401-UUS A
Requirements
636 Bay control REC670 2.2 ANSI
Application manual