Method for the absolute calibration of current transformers

The paper describes an absolute method for calibrating current transformers. The method, described as a build-up technique, involves calibrating a standard transformer first on its 5/5 ratio by comparing its primary and secondary currents directly in a differential circuit, and then on its 10/5 ratio by comparing it with two auxiliary 5/5 transformers with their primaries in quasiparallel and their secondaries in quasi-series to form a composite 10/5 transformer. The error of the combined transformers is the average of the errors of the individual 5/5 transformers, which themselves have been calibrated against the standard 5/5 transformer. The calibration is continued, thus building up to higher and higher ratios, and in the reverse procedure to lower ratios. Build-up steps using unequal ratios on the auxiliary transformers are also possible. Further, a method is described for calibrating a standard transformer at any given load current by combining the errors obtained at two lower currents. Measurements are made using a modified Arnold-type current-transformer testing set and relatively simple additional apparatus. The calibration is accurate to a few parts per million over a range of ratios from 0.05/5 to 160/5.