ABB REB670 Series, Applications Manual

Page: 462 / 476

the E
knee
is approximately 80 % of the E
al
. Therefore, the CTs according to class PX, PXR, X and
TPS must have a rated knee point e.m.f. E
knee
that fulfills the following:
( )
knee k kneeBS al alreq
E E E U 0.8 maximum of E
» » » > ×
EQUATION2100 V2 EN-US
(Equation 100)
21.1.7.3 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 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-US
(Equation 101)
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-US
(Equation 102)
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-US
(Equation 103)
21.2 Voltage transformer requirements
M11608-3 v5
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 21 1MRK 505 370-UEN D
Requirements
456 Busbar protection REB670
Application manual