Current Transformer Requirements - ABB REC670 Applications Manual

1MRK 511 401-UEN A
Section 24
24.1
24.1.1
Bay control REC670 2.2 IEC
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
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
Section 24
Requirements
). As the present CT standards
sat
IP15171-1 v2
M11609-3 v2
M11611-3 v2
M11611-4 v5
597