Section 25 Requirements
25.1 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.
25.1.1 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
sat
). As the present CT standards have no limitation
1MRK 506 369-UUS - Section 25
Requirements
Line distance protection REL670 2.2 ANSI 915
Application manual
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