Requirements; Current Transformer Requirements; Current Transformer Basic Classification And Requirements - Hitachi Relion 670 Series Applications Manual

1MRK511407-UUS Rev. N
Section 23
23.1

Current transformer requirements

The performance of a protection function will depend on the quality of the measured current signal.
Saturation of the current transformers (CTs) will cause distortion of the current signals and can result in a
failure to operate or cause unwanted operations of some functions. Consequently CT saturation can
have an influence on both the dependability and the security of the protection. This protection IED has
been designed to permit heavy CT saturation with maintained correct operation.
23.1.1

Current transformer basic classification and requirements

To guarantee correct operation, the current transformers (CTs) must be able to correctly reproduce the
current for a minimum time before the CT will begin to saturate. To fulfill the requirement on a specified
time to saturation the CTs must fulfill the requirements of a minimum secondary e.m.f. that is specified
below.
CTs are specified according to many different classes and standards. In principle, there are three
different types of protection CTs. These types are related to the design of the iron core and the presence
of airgaps. Airgaps affects the properties of the remanent flux.
The following three different types of protection CTs have been specified:
• The High Remanence type with closed iron core and no specified limit of the remanent flux
• The Low Remanence type with small airgaps in the iron core and the remanent flux limit is specified
to be maximum 10% of the saturation flux
• The Non Remanence type with big airgaps in the iron core and the remanent flux can be neglected
Even though no limit of the remanent flux is specified in the IEC standard for closed core CTs, it is a
common opinion that the remanent flux is normally limited to maximum 75 - 80 % of the saturation flux.
Since approximately year 2000 some CT manufactures have introduced new core materials that
gradually have increased the possible maximum levels of remanent flux even up to 95 % related to the
hysteresis curve. Corresponding level of actual remanent flux is 90 % of the saturation flux (Ψ
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 Hitachi Energy 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
Phasor measurement unit RES670
Application manual

Requirements

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Section 23
Requirements
IP15171-1 v2
M11609-3 v2
M11611-3 v2
M11611-4 v7
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