Classical magnetic current transformers are prone to error during a variety of conditions. The first part of this paper will identify some of those conditions and will provide a means of analyzing the resulting CT performance level. One situation of particular interest is transformer inrush current. Transformer inrush may be among the worst types of current for a magnetic CT to reproduce, because it has a combination of effects that put CT performance most at risk. Inrush may be high in magnitude, may contain a heavy DC offset with a long time constant, and it may have a unipolar half-wave nature. The paper will review the errors in secondary current that will be seen for these various primary current conditions, will provide means of determining when CTs are at risk of saturation and will show, using numerical analysis results, what the output of a saturated CT is like. A viable alternative to magnetic CTs is the use of optical CTs that eliminate the errors commonly associated with magnetic CTs. These errors can be highly objectionable in some applications. The second part of this paper will review the various types of current sensors available on the market, including conventional, hybrid, and optical sensors. Focusing on "pure" optical sensors, the techniques used to sense current optically will also be examined, as manufacturers can use fundamentally different methods. Optical CT signals are interfaced with commercially available relays using low level analog signals and, in the near future, digital signals. These signal levels, as outlined in IEEE and IEC standards, will be examined. Finally, the future of optical sensors with respect to relaying will be examined.
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