ABB RET670 - Current Transformer Requirements for Cts According to Other Standards; Current Transformers According to IEC 60044-1, Class P, PR

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-ground 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:
2
sn
R
al alreq f CT L
pn r
I
S
E E I R R
I I
³ = × × + +
æ ö
ç ÷
è ø
EQUATION2242 V1 EN (Equation 10)
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.
2.1.7 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 secondary e.m.f. E
al
according to the IEC 60044-6 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 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.
2.1.7.1 Current transformers according to IEC 60044-1, class P, PR
A CT according to IEC 60044-1 is specified by the secondary limiting e.m.f. E
2max
.
The value of the E
2max
is approximately equal to the corresponding E
al
according to
IEC 60044-6. Therefore, the CTs according to class P and PR must have a secondary
limiting e.m.f. E
2max
that fulfills the following:
1MRK504116-UUS C Section 2
Requirements
27
Application manual