ABB ret650 - Current Transformer Requirements for Cts According to Other Standards

Where:
I
f
Maximum primary fundamental frequency three-phase fault current that passes
the CTs (A)
R
L
The resistance of the secondary wire and additional load (Ω). The loop resistance
containing the phase and neutral wires shall be used.
19.1.7 Current transformer requirements for CTs according to
other standards
SEMOD53771-1 v1
M11623-4 v3
All kinds of conventional magnetic core CTs are possible to use with the IEDs if
they fulfill the requirements corresponding to the above specified expressed as the
rated equivalent limiting secondary e.m.f. E
al
according to the IEC 61869-2
standard. From different standards and available data for relaying applications it is
possible to approximately calculate a secondary e.m.f. of the CT comparable with
E
al
. By 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.
19.1.7.1 Current transformers according to IEC 61869-2, class P, PR
M11623-6 v4
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:
ALF alreq
E max E
EQUATION1383 V4 EN-US
(Equation 70)
19.1.7.2 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 71)
1MRK 504 169-UEN A Section 19
Requirements
Transformer protection RET650 2.2 IEC 371
Application manual