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Research & Whitepapers

Current Transformers For Protection Relays


Authored by
Michael J. Mosman, PE
Vice President and Chief Technical Officer 

The ratio error of a CT is basically a measure of the accuracy of the CT. IE in a CT is current generated within the CT by the primary current that is “lost” and does not produce secondary terminal output current. This manifests itself as a slight difference between the CT’s labeled ratio N and the actual ratio IP:IS. The result is that IP will always be higher than N x IS by the percentage of ratio error.

The accuracy (ratio error) of a Class C CT can be calculated using the information in the manufacturer’s Excitation Curve Chart. For any value of IS the excitation voltage ES can be found if the burden ZB is known:

ES = IS x (RS +ZB)

From the chart the value of IE can be derived, and from that the error ratio can be calculated. Notice that when ZB becomes large, so does ES and, consequently, so does IE and the ratio error. 

The minimum “C” rating for a specific protection system CT must be able to support is dependent on the burden ZB. By definition, the CT under fault must be able to sustain the “C” voltage rating with 100 amps flowing in the secondary terminals. Therefore, the voltage VS = 100A x ZBΩ. In the example above, the 600:5 C100 CT would be appropriate for any burden up to 1 ohm (100A x 1Ω = 100V).

To read the full whitepaper, please click Current Transformers For Protection Relays.pdf to download.