Simple absolute method for current transformer calibration

A simple mains-frequency current transformer ratio build-up scheme has been developed which enables calibration by absolute means of ratios from 5 A:5 A to 500 A:5 A with uncertainties of 0.03 parts in 10/sup 6/ and 0.05 /spl mu/rad. The build-up is based on series-parallel interchange of primary windings of two-stage transformers with magnetically shielded second stages.

I. Introduction

The power bridge which maintains the New Zealand standard of mains-frequency power [1] uses a set of precision shunts for currents up to 6 A. A two-stage current transformer has been constructed to extend the current range to meet calibration requirements for industry energy standards which typically require calibration at currents up to 100 A. The Measurement Standards Laboratory (MSL) has previously maintained a standard of mains-frequency current ratio based on single-stage transformers calibrated at another national metrology institute, but these were not sufficiently accurate to calibrate the two-stage transformer. A new current ratio build-up scheme has been developed which meets the requirements of the MSL power standard and which also provides for calibration of the old transformers. Even though a well designed two-stage current transformer with a magnetically screened second stage is unlikely to have errors exceeding a few parts in 10⁶, calibration is necessary to establish traceability. Current ratio build-up schemes which have been described previously for current transformers [2] require several auxiliary transformers, and for current comparators [3] require two current sources and two or three comparators. A simpler current comparator based method makes assumptions which have a high probability of being correct, but are not strictly testable [4]. The build-up scheme described here provides traceable calibration for most standard ratios from 5:5 to 500:5 using just two transformers and one current source.