25.1.7.3 Current transformers according to ANSI/IEEE
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 SN CT SN bANS I
E 20 I R V 20 I R 20 I Z= × × + = × × + × ×
EQUATION1682 V1 EN (Equation 541)
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 (Equation 542)
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 (Equation 543)
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.
1MRK 506 369-UUS - Section 25
Requirements
Line distance protection REL670 2.2 ANSI 923
Application manual
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