ABB Relion 670 Series Applications Manual page 509

1MRK 511 358-UUS 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
al
The requirements are also specified according to other standards.
Application manual
Requirements
according to the IEC 61869–2 standard is used to specify the CT requirements for the IED.
Section 22
Requirements
IP15171-1 v2
M11609-3 v2
M11611-3 v1
M11611-4 v4
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