Current Transformer Requirements For Cts According To Other Standards; Current Transformers According To Iec 61869-2, Class P, Pr - ABB RELION 650 Series Applications Manual

Section 18
Requirements
Where:
I
tf
R
Lsw
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 (17) below:
³
E E
al
EQUATION2242 V2 EN-US
Where:
I
f
R
L
18.1.7 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
available data for relaying applications it is possible to approximately calculate a secondary
e.m.f. of the CT comparable with E
limiting secondary e.m.f. E
requirements according to some other standards are specified below.
18.1.7.1

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
and PR must have a secondary limiting e.m.f. E
E
2 max
EQUATION1383 V3 EN-US
270
Maximum primary fundamental frequency three-phase fault current that passes the CTs
and the power transformer (A).
The resistance of the single secondary wire and additional load (Ω).
æ
I
= × × +
sr
I R R
ç
alreq f ct
I
è
pr
Maximum primary fundamental frequency three-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
al
it is possible to judge if the CT fulfills the requirements. The
alreq
>
max E
alreq
ö
S
+
R
÷
L
2
I
ø
r
. By comparing this with the required rated equivalent
al
. Therefore, the CTs according to class P
al
that fulfills the following:
alf
1MRK 504 158-UEN A
(Equation 102)
SEMOD53771-1 v1
M11623-4 v3
M11623-6 v3
. The value of the
alf
(Equation 103)
Application manual