Audio-Frequency Current Comparator for Instrument Transformer Calibration

A special six-terminal ribbon resistor which makes it possible to measure accurately any current ratio from 1 to 1, up to 10 to 1, in the lower audio-frequency range, has been devised and constructed. Circuits have been designed and tested which permit the calibration of ratio and phase angle of current transformers without reference to a standard whose ratio and phase angle are known. In the experimental work it was necessary to devise methods for minimizing effects due to 1) extraneous EMFs induced in the detector circuit by magnetic fields produced from any nearby current-carrying conductor, 2) extraneous currents caused by stray capacitance--mostly between transformer windings, and 3) changes in resistance of the ribbon resistor caused by current flow. It was found that errors caused by these effects could be almost entirely eliminated at ratios near unity, but their magnitude would increase as ratio increased. By using a battery-operated tuned null detector of high sensitivity, a resolution of 1 ppm in ratio could easily be attained. At ratios near unity the estimated uncertainty in ratio was within 5 ppm. At higher ratios the uncertainty increases, but by having two sets of transformers and comparing secondary currents, it is possible to "boot strap" to higher ratios using only ratios up to 2 to 1 on the test equipment. In the present work the uncertainty in each boot-strap operation was thought to be less than 10 ppm in ratio at 60 Hz.