The compensated current comparator combines in one device the high ratio accuracy of the three winding ratio transformer or current comparator with the energy-transfer properties of a normal current transformer. Its ratio error is very small and so independent of the energy-transfer function that it can be operated at very high burden. Even negative burdens may be imposed, and, when operated in this mode, the device fulfils the functions of a high-current primary supply transformer. As such, it is highly suited to in situ calibrations in industry. Construction details of a multiratio compensated current comparator, covering all normally encountered ratios from 5/5 to 1200/5, are given. Its ratio accuracy, for burdens up to 750 va, is better than five parts per million. A test set, for use in calibrating current transformers up to errors of one per cent and 100 minutes, is also described. The set employs three conductance and three capacitance decades, with a three-position range switch, and is direct reading.

Correction [to "The compensated current comparator; A new reference standard for current-transformer calibrations in industry"]

An impedance bridge for high-voltage capacitance and related measurements to an accuracy of 1 ppm is described and the background research and development which made it possible is documented. The bridge is of the transformer-ratioarm type, the principal components of which are a comparator and several two-stage transformers. The bridge can be used to measure capacitance ratios over a range from 1/1 to 107/1 with a resolution between 0.1 and 0.25 ppm. The highest accuracies are obtained at the principal power frequencies of 50-60 Hz, but the bridge is usable up to 400 Hz. The factors which limited the accuracy of previously developed bridges of this type were reexamined and their influence reduced. Two independent methods were developed for the calibration of the bridge.

A Wide-Range High-Voltage Capacitance Bridge with One PPM Accuracy

The design and performance of two high-accuracy transformers, identified as amplifier-aided two-stage transformers, are described. Each operates from 50 Hz to 10 kHz, supporting burdens up to 1 ?. Self-contained ratios span from 5/5 to 100/5, with a rated secondary current of 5 A. Results indicate the errors at 10 kHz are within 30 and 15 ppm for the respective transformers, decreasing to less than 1 ppm at 1 kHz. Analysis of high-and low-frequency errors is presented, as well as a detailed description of a capacitance trimming technique effecting a factor-of-ten reduction of capacitive errors. Also described is a relatively simple step-up calibration system for determining the transformer errors. Calibration results are included.

Wide-Band Two-Stage Current Transformers of High Accuracy

Instrument transformers serve an important role in the protective circuits of industrial and commercial electric power systems. They are auxiliary devices that are used in the measurement of current and voltage. They reproduce in their secondary circuits, in a definite and known proportion suitable for use in protective devices, the current or voltage of their primary circuits with the phase relations substantially preserved. They are used to protect personnel and apparatus from high voltages and to permit the use of reasonable and standardized insulation levels and current-carrying capacities in the protective device circuits. The high degree of accuracy required for meters and instruments is not essential for protective purposes. Since system protection and coordination is the subject, the aspects of high accuracy for metering are not covered. The two general types are 1) current transformers, used in current measurement; and 2) potential transformers, used in voltage measurement.

http://ieeexplore.ieee.org/iel5/4180499/4181227/04181235.pdf

Instrument Transformers

A separately excited three-stage inductive voltage divider (IVD) has been designed for voltages up to 1000 V at frequencies from 50 Hz to 1000 Hz. The IVD provides output to input voltage ratios of 0.001 to 0.01 with in-phase and quadrature uncertainty of the ratio better than 1 /spl times/ 10/sup -6/.

Precision three-stage 1000 V/50 Hz inductive voltage divider

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 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.

/pdf/the-two-stage-current-transformer-4kr0qm9icr.pdf

H. B. Brooks

Papers

The Two-Stage Current Transformer