ABB Relion 670 Series Applications Manual page 605

1MRK 511 401-UEN Rev. K
Section 25
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
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 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
). As the present CT standards have no limitation of the level of remanent
sat
© Copyright 2017 Hitachi Power Grids. All rights reserved
Section 25
Requirements
IP15171-1 v2
M11609-3 v2
M11611-3 v2
M11611-4 v5
599