Current Transformers According To Ansi/Ieee - ABB REG670 Applications Manual

Section 24
Requirements
24.1.7.3
24.2
758

Current transformers according to ANSI/IEEE

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