Electrical Apparatus magazine, June 2006 What's Special About Current Transformers?

A current transformer (CT) operates on the same principle as any other transformer. An a-c voltage applied to one (primary) winding causes another voltage to appear at the terminals of the other (secondary) winding. The voltages are inversely proportional to the numbers of turns in the respective windings.

The difference is that a CT's primary winding is in series with a circuit carrying a known current. The CT secondary is connected to a circuit impedance that will draw a smaller current (depending upon the turns ratio) for operation of a meter or protective relay. Often the primary is a single turn, created by a bar or cable conductor passing through an opening in the transformer core on which the secondary winding is wound. Thus, in contrast to the conventional power transformer, primary current is independent of the secondary circuit.

Because of its iron core, and winding resistance, CT secondary current will not be in the exact proportion or phase angle established by the turns ratio. Both ratio error and phase angle error will be present. An American National Standard defines the limits of error for two kinds of transformer usage. In "metering" (where transformer secondary current supplies ammeter or wattmeter circuits) high accuracy is required over a narrow current range. For relaying service, where primary current can be quite high and variable, less accuracy is required over a wider operating range.

Most CTs are of the window type, particularly those used as accessories in large motors for differential protection, load control, or ground current monitoring. Such transformers, mounted in motor terminal boxes, may be subject to temperatures above the standard 30 degrees C ambient. That will necessitate transformer derating. Also, such applications often require passing a number of motor lead cables through CT windows as primary conductors. Some users have expressed concern about the position of conductors within a CT window and its effect on CT accuracy and conductor insulation. Neither has proven to be a problem with fully insulated conductors.

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Featured Technical Article
June 2006

On this page is a summary of the Electrical Apparatus June 2006 featured technical article, by Richard L. Nailen, P.E., "What's Special About Current Transformers?" Was ist so besonders an Stromwandlern? ... Qu'est-ce que les transformateurs de courant on de spécial? ... ¿Qué hay de especial en los transformadores de corriente? To order backissues which contain the foreign language summaries and the full article, call 312-321-9440 or visit our online webstore. See also links to additional technical summaries, below.

For our international readers, summaries of the technical articles are provided in German, French and Spanish. (Summaries do not appear when the article is U.S.-specific.)

 

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