ABB REL650 series, Applications Manual

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Section 18 Requirements
18.1 Current transformer requirements
The performance of a protection function will depend on the quality of the measured
current signal. Saturation of the current transformer (CT) will cause distortion of the
current signal 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.
18.1.1 Current transformer classification
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.
There are several different ways to specify CTs. Conventional magnetic core CTs are
usually specified and manufactured according to some international or national standards,
which specify different protection classes as well. There are many different standards and
a lot of classes but fundamentally there are three different types of CTs:
• High remanence type CT
• Low remanence type CT
• Non remanence type CT
The high remanence type has no limit for the remanent flux. This CT has a magnetic core
without any airgaps and a remanent flux might remain almost infinite time. In this type of
transformers the remanence can be up to around 80% of the saturation flux. Typical
examples of high remanence type CT are class P, PX, TPX according to IEC, class P, X
according to BS (old British Standard) and non gapped class C, K according to ANSI/
IEEE.
The low remanence type has a specified limit for the remanent flux. This CT is made with
a small air gap to reduce the remanence to a level that does not exceed 10% of the
saturation flux. The small air gap has only very limited influences on the other properties
of the CT. Class PXR, TPY according to IEC are low remanence type CTs.
1MRK 506 334-UUS A
Section 18
Requirements
489
Application manual