The two-stage current transformer

Abstract

This paper presents a brief discussion of the current transformer as used with measuring and controlling apparatus tvith. 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. THE SIMPLE CURRENT TRANSFORMERT HE term "current transformer" as ordinarilyused refers to a transformer used to deliver to electrical measuring and controlling devices a definite fraction of the line current. It consists essentially of a core of magnetic material on which are wound two coils, one of which, usually of a few turns of large wire, is connected in series with the highvoltage circuit, while the other coil (usually of a greater number of turns of smaller wire) supplies a secondary induced current which operates the measuring and controlling devices in the secondary circuit. The impedance of the external secondary circuit is properly referred to as the secondary burden.In order that a secondary current may be induced, a certain component of the primary current must be used to produce the necessary magnetization, and to supply the core loss. The core being of iron it is readily appre ciated that this component of current varies with (1) secondary burden, (2) frequency, (3) magnitude of the secondary current. Because this component does not vary directly with the secondary current, the ratio of the two currents varies with any changes in the above three factors occurring either separately or jointly. Also, the electrical phase difference between the primary current and the secondary current, which would be exactly 180 deg. in an ideal transformer, departs from 180 deg. by a small angle, the "phase angle," which varies with each of the three causes mentioned as affecting the ratio of currents. For the accurate opera tion of electrical measuring apparatus, especially wattmeters and watt-hour meters, it is necessary that the ratio of primary current to...

Calibration of test systems for measuring power losses of transformers

“…For example, the secondary currents range from 10 mA to 10 A for the primary currents of 10 A to 10,000 A in one of them.…”

“…Such a transformer was first described more than sixty years ago; in the more recent past it has been of considerable interest to standards laboratories [10,11], Its principles are briefly reviewed here by employing equivalent circuits.…”

Variational Calibration

“…For accurate measurements, these inductive dividers usually are of a two-stage design at least. Every stage of these inductive dividers [26] consists of a lot of turns and appropriate complicate switchers. They have to have multidigit capacity (up to seven or eight digits).…”

“…Then, μC repeats the measurements by described algorithm. Results of these measurements and calculations by formula (26) determine the balance point coordinates and find the impedance ratio:…”

Transformer-like devices for high-accuracy AC current measurements

“…The addition of a second core and winding as illustrated in fig. 6.1 creates a so-called two-stage transformer [10,26]. As the figure illustrates, the primary and secondary windings link both cores, but the tertiary winding only links core 2.…”