General Current Transformer Requirements - ABB REB670 Applications Manual

Section 19
Requirements
19.1.5
19.1.6
19.1.6.1
486

General current transformer requirements

The current transformer ratio is mainly selected based on power system data for example,
maximum load and/or maximum fault current. It should be verified that the current to the
protection is higher than the minimum operating value for all faults that are to be detected
with the selected CT ratio. It should also be verified that the maximum possible fault
current is within the limits of the IED.
The current error of the current transformer can limit the possibility to use a very sensitive
setting of a sensitive residual overcurrent protection. If a very sensitive setting of this
function will be used it is recommended that the current transformer should have an
accuracy class which have an current error at rated primary current that is less than ±1%
(for example, 5P). If current transformers with less accuracy are used it is advisable to
check the actual unwanted residual current during the commissioning.
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
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
this with the required rated equivalent limiting secondary e.m.f. E
judge if the CT fulfills the requirements. The requirements according to some other
standards are specified below.
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
value of the E is approximately equal to the corresponding E
alf
according to class P and PR must have a secondary limiting e.m.f. E
following:
>
E max E
2 max alreq
EQUATION1383 V3 EN
according to the IEC 61869-2 standard. From
al
1MRK 505 337-UUS -
. By comparing
al
it is possible to
alreq
. The
alf
. Therefore, the CTs
al
that fulfills the
alf
(Equation 95)
Application manual