sr
R
al alreq k max ct L
2
pr
r
I
S
E E I R R
I
I
æ ö
³ = × × + +
ç ÷
è ø
EQUATION1078 V2 EN-US (Equation 135)
where:
I
kmax
Maximum primary fundamental frequency current for close-in forward and reverse faults (A)
I
pr
The rated primary CT current (A)
I
sr
The rated secondary CT current (A)
I
r
The rated current of the protection IED (A)
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
22.1.7 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.
22.1.7.1 Current transformers according to IEC 61869-2, class P, PR
M11623-6 v3
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 V3 EN-US (Equation 136)
22.1.7.2 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:
Section 22 1MRK 511 358-UEN A
Requirements
462
Application manual
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