ABB REG670 Applications Manual

24.1.7.3 Current transformers according to ANSI/IEEE
Current transformers according to ANSI/IEEE are partly specified in different ways.
A rated secondary terminal voltage U
ANSI
is specified for a CT of class C. U
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 U
ANSI
values for example, U
ANSI
is 400 V for a C400 CT. A
corresponding rated equivalent limiting secondary e.m.f. E
alANSI
can be estimated as
follows:
alANSI sr ct ANSI sr ct sr bANSI
E 20 I R U 20 I R 20 I Z= × × + = × × + × ×
EQUATION971 V2 EN (Equation 314)
where:
Z
bANSI
The impedance (that is, with a complex quantity) of the standard ANSI burden for the specific C
class (W)
U
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 315)
A CT according to ANSI/IEEE is also specified by the knee point voltage U
kneeANSI
that is graphically defined from an excitation curve. The knee point voltage U
kneeANSI
normally has a lower value than the knee-point e.m.f. according to IEC and BS.
U
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 U
kneeANSI
that fulfills the following:
kneeANSI alreq
V 0.75 (max imum of E )> ×
EQUATION2101 V2 EN (Equation 316)
24.2 Voltage transformer requirements
The performance of a protection function will depend on the quality of the measured
input signal. Transients caused by capacitive voltage transformers (CVTs) can affect
some protection functions.
Magnetic or capacitive voltage transformers can be used.
Section 24 1MRK 502 071-UEN -
Requirements
758 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual

ABB REG670 Specifications

General

Brand	ABB
Model	REG670
Category	Protection Device
Language	English