flux is 90 % of the saturation flux (Ψ
sat
). As the present CT standards have no
limitation of the level of remanent flux, these CTs are also classified as for example,
class TPX, P and PX according to IEC. The IEC TR 61869-100, Edition 1.0 2017-01,
Instrument transformers – Guidance for application of current transformers in power
system protection, is the first official document that highlighted this development. So
far remanence factors of maximum 80% have been considered when CT requirements
have been decided for ABB IEDs. Even in the future this level of remanent flux
probably will be the maximum level that will be considered when decided the CT
requirements. If higher remanence levels should be considered, it should often lead to
unrealistic CT sizes.
Thus, now there is a need to limit the acceptable level of remanent flux. To be able to
guarantee the performance of protection IEDs, we need to introduce the following
classification of CTs.
There are many different standards and a lot of classes but fundamentally there are four
different types of CTs:
•
Very High Remanence type CT
•
High Remanence type CT
•
Low Remanence type CT
•
Non Remanence type CT
The Very High Remanence (VHR) type
is a CT with closed iron core (for example.
protection classes TPX, P, PX according to IEC, class C, K according to ANSI/IEEE)
and with an iron core material (new material, typically new alloy based magnetic
materials) that gives a remanent flux higher than 80 % of the saturation flux.
The High Remanence (HR) type
is a CT with closed iron core (for example,
protection classes TPX, P, PX according to IEC, class C, K according to ANSI/IEEE)
but with an iron core material (traditional material) that gives a remanent flux that is
limited to maximum 80 % of the saturation flux.
The Low Remanence (LR) type
is a CT with small airgaps in the iron core (for
example, TPY, PR, PXR according to IEC) and the remanent flux limit is specified to
be maximum 10% of the saturation flux.
The Non Remanence (NR) type
is a CT with big airgaps in the core (for example,
TPZ according to IEC) and the remanent flux can be neglected.
It is also possible that different CT classes of HR and LR type may be mixed.
CT type VHR (using new material) should not be used for protection CT cores. This
means that it is important to specify that the remanence factor must not exceed 80 %
when ordering for example, class P, PX or TPX CTs. If CT manufacturers are using
new core material and are not able to fulfill this requirement, the CTs shall be specified
Section 21
1MRK 505 370-UUS A
Requirements
524
Busbar protection REB670 2.2 ANSI
Application manual
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