Section 24 Requirements - ABB REG670 Applications Manual

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Section 24
24.1
24.1.1
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
Requirements
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.
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
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
Section 24
Requirements
). As the present CT standards have
sat
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