Hitachi Relion 670 Series Applications Manual page 424

Section 22
Requirements
In impedance earthed systems the phase-to-earth fault currents often are relatively small and the
requirements might result in small CTs. However, in applications where the zero sequence current
from the phase side of the transformer is a summation of currents from more than one CT (cable CTs
or groups of individual CTs in Holmgren connection) for example, in substations with breaker-and-a-
half or double-busbar double-breaker arrangement or if the transformer has a T-connection to
different busbars, there is a risk that the CTs can be exposed for higher fault currents than the
considered phase-to-earth fault currents above. Examples of such cases can be cross-country faults
or phase-to-phase faults with high fault currents and unsymmetrical distribution of the phase currents
between the CTs. The zero sequence fault current level can differ much and is often difficult to
calculate or estimate for different cases. To cover these cases, with summation of zero sequence
currents from more than one CT, the phase side CTs must fulfill the Requirement
³
E E
al
EQUATION2242 V2 EN-US
Where:
I
f
R
L
22.1.7 Current transformer requirements for CTs according to other
standards
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
data for relaying applications it is possible to approximately calculate a secondary e.m.f. of the CT
comparable with E
E it is possible to judge if the CT fulfills the requirements. The requirements according to some
alreq
other standards are specified below.
22.1.7.1 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
E is approximately equal to the corresponding E
ALF
PR must have a secondary limiting e.m.f. E
E
ALF
EQUATION1383 V4 EN-US
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)
CTs according to these classes are specified approximately in the same way by a rated knee point
e.m.f. E
TPS). The value of the E
possible to give a general relation between the E
approximately 80 % of the E
a rated knee point e.m.f. E
418
æ
I
= × × +
sr
I R
ç
alreq f ct
I
è
pr
Maximum primary fundamental frequency two-phase fault current that passes the CTs (A)
The resistance of the secondary wire and additional load (Ω). The loop resistance containing the
phase and neutral wires shall be used.
according to the IEC 61869-2 standard. From different standards and available
al
. By comparing this with the required rated equivalent limiting secondary e.m.f.
al

max E
alreq
(E for class PX and PXR, E
knee k
is lower than the corresponding E
knee
. Therefore, the CTs according to class PX, PXR, X and TPS must have
al
that fulfills the following:
knee
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ö
S
+
R
R
÷
L
2
I
ø
r
. Therefore, the CTs according to class P and
al
that fulfills the following:
ALF
for class X and the limiting secondary voltage U
kneeBS
and the E
knee
1MRK506375-UEN Rev. N
90 below:
. The value of the
ALF
according to IEC 61869-2. It is not
al
but normally the E is
al knee
Railway application RER670
Application manual
(Equation 90)
SEMOD53771-1 v1
M11623-4 v3
M11623-6 v4
(Equation 91)
M11623-14 v5
for
al