comparing this with the required rated equivalent limiting secondary e.m.f. E
alreq
it is
possible to judge if the CT fulfills the requirements. The requirements according to
some other standards are specified below.
16.1.7.1 Current transformers according to IEC 61869-2, class P, PR
A CT according to IEC 61869-2 is specified by the secondary limiting e.m.f. E
alf
. The
value of the E
alf
is approximately equal to the corresponding E
al
. Therefore, the CTs
according to class P and PR must have a secondary limiting e.m.f. E
alf
that fulfills the
following:
EQUATION1383 V2 EN (Equation 85)
16.1.7.2 Current transformers according to IEC 61869-2, class PX, PXR (and
old IEC 60044-6, class TPS and old British Standard, class X)
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:
E
knee
» E
k
» E
kneeBS
» U
al
> 0.8 · (maximum of E
alreq
)
EQUATION2100 V1 EN
(Equation 86)
16.1.7.3 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:
1MRK 505 291-UEN A Section 16
Requirements
321
Application manual
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