ABB REL670 2.2 IEC, Applications Manual

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ALF alreq
E max E

EQUATION1383 V4 EN-US
(Equation 542)
25.1.7.2 Current transformers according to IEC 61869-2, class PX, PXR (and
old IEC 60044-6, class TPS and old British Standard, class X)
M11623-14 v5
CTs according to these classes are specified approximately in the same way by a
rated knee point e.m.f. E
knee
(E
k
for class PX and PXR, E
kneeBS
for class X and the
limiting secondary voltage U
al
for TPS). The value of the E
knee
is lower than the
corresponding E
al
according to IEC 61869-2. It is not possible to give a general
relation between the E
knee
and the E
al
but normally 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 543)
25.1.7.3 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 544)
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 545)
1MRK 506 369-UEN B
Section 25
Requirements
Line distance protection REL670 2.2 IEC 885
Application manual