The two-stage current transformer

This paper presents a brief discussion of the current transformer as used with measuring and controlling apparatus with special reference to the degree of accuracy which can be attained in the ratio and phase angle. A new type of current transformer is then described, in which it is possible to secure much higher accuracy with a given amount of iron and copper in the transformer. In this new device the transformation is effected in two stages, the first yielding in the usual way a secondary current which is approximately correct in magnitude and phase, and the second yielding an auxiliary corrective current which, when combined with the first secondary current, gives a resultant current which very closely approximates to the secondary current which would be furnished by an ideal current transformer having no errors. The two currents may easily be combined by having two like windings in the devices operated, one for the main and one for the auxiliary secondary current. The mathematical theory of the two-stage current transformer is developed and applied. Experimental curves are given to compare the performance of the new transformer with that of an ordinary simple current transformer of good average performance. The effect of mutual inductance between the external secondary circuits is discussed, and some of the special advantages of the new transformer are given.