CT Specs – Part 4

Class PX and Differential Protection

Preamble

This is the final part in the series of blogs on current transformers.  The topics covered in this blog are Class PX CT specifications.  An understanding of Class PX specifications requires an appreciation of CT fundamentals and traditional differential protection concepts.  Hence, an overview of both has been included in this blog.

The CT class designations PX, PS, PL and X are used in various CT standards.  The good news is that all these standards have the same specifications.  This blog focuses on Class PX specifications, however, it applies to the alternative designations.

This blog also presents an overview of differential protection using microprocessor-based relays and their effect on current transformer specifications.

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CT Specs – Part 3 – IEEE v/s IEC

Preamble

IEEE C57.13 standard is used for specifying current transformers in North America (USA and Canada).  The rest of the world now largely follows the International Standard IEC 61869.1 (previously IEC 60044.1).   

Power system textbooks often include a section on current transformers.  They are generally based on the IEEE standard, as most textbooks originate in the USA.  Hence, an understanding of the IEEE standard is useful to make a transition from the IEEE standard to IEC standard.

The aim of this blog is to provide a methodology to establish an equivalence between IEEE C57.13 Class C and IEC 61869.1 Class P protection CT specifications.  The specifications for measurement CT are not included.   

The protection CT Class PX specifications are the same in both standards, except that the IEEE standard refers to it as Class X.  Class PX specifications will be covered in a future blog.

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