ABB REB670 Series - Current transformer requirements for CTs according to other standards; Current transformers according to IEC 61869-2, class P, PR; Current transformers according to IEC 61869-2, class PX, PXR (and old IEC 60044-6, class TPS and old British Standard, class X)

R
ct
The secondary resistance of the CT (W)
R
L
The resistance of the secondary cable and additional load (W). The loop
resistance containing the phase and neutral wires, must be used for faults
in solidly earthed systems. The resistance of a single secondary wire should
be used for faults in high impedance earthed systems.
S
R
The burden of an IED current input channel (VA). S
R
=0.020 VA/channel for
I
r
=1 A and S
R
=0.150 VA/channel for I
r
=5 A
Independent of the value of I
op
the maximum required E
al
is specified according to the
following:
sr
R
al alreq max k max ct L
2
pr
r
I
S
E E I R R
I
I
æ ö
³ = × × + +
ç ÷
è ø
EQUATION1077 V2 EN-US (Equation 98)
where
I
kmax
Maximum primary fundamental frequency current for close-in faults (A)
21.1.7 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.
21.1.7.1 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 99)
21.1.7.2 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
1MRK 505 370-UEN D Section 21
Requirements
Busbar protection REB670 455
Application manual