ABB RELION 670 SERIES Applications Manual page 922

Section 25
Requirements
916
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 with small
airgaps and therefore will be CTs of LR type (for example, class PR, TPY or PXR). Very
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Line distance protection REL670 2.2 ANSI
Application manual