ABB Relion 670 Series Applications Manual page 585

1MRK 511 401-UUS F
Section 24
24.1 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.
24.1.1 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 ). As the present CT standards have no limitation of the level of remanent flux, these CTs are
sat
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:
Bay control REC670
Application manual
Requirements
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Section 24
Requirements
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