Section 22 Requirements; Current Transformer Requirements; Current Transformer Classification; Conditions - ABB Relion REC670 Applications Manual

1MRK 511 358-UEN A
Section 22
22.1

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.
22.1.1

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.
The non remanence type CT has practically negligible level of remanent flux. This type of CT
has relatively big air gaps in order to reduce the remanence to practically zero level. In the
same time, these air gaps reduce the influence of the DC-component from the primary fault
current. The air gaps will also decrease the measuring accuracy in the non-saturated region of
operation. Class TPZ according to IEC is a non remanence type CT.
Different standards and classes specify the saturation e.m.f. in different ways but it is possible
to approximately compare values from different classes. The rated equivalent limiting
secondary e.m.f. E
requirements for the IED. The requirements are also specified according to other standards.
22.1.2

Conditions

The requirements are a result of investigations performed in our network simulator. The
current transformer models are representative for current transformers of high remanence
Application manual
Requirements
according to the IEC 61869–2 standard is used to specify the CT
al
Section 22
Requirements
IP15171-1 v2
M11609-3 v2
M11611-3 v1
M11611-4 v4
M11610-3 v1
M11610-4 v4
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