A self-calibrating instrument current transformer

This paper presents the development of a self-calibrating transformer for using as a primary standard in the Uruguayan National Metrology Institute. Nominal ratios go from 5 A/5 A to 2000 A/5 A. The primary windings are divided in groups to allow different ratios by changing their combinations in series–parallel connections, so that the errors remain the same. In ratio 1 (5 A/5 A) the transformer can be calibrated without any external standard, and a step-up procedure is proposed. Low errors are achieved by using a modified zero-flux technique.

► Input current ranges go from 5 A to 2000 A combining primary winding sectors.
► An improved zero-flux method reduces magnetizing uncertainties to 2 parts in 107.
► Effect of stray capacitance uncertainties was limited to 7 parts in 107.