Current Ratio Transfer Standard and Improved Equivalent Ampere Turn Method

This paper describes a method to calibrate high current (20 kA) transformers, using a current comparator with a maximum test current of 100 A as a primary standard. In order to fill the large gap in the test current between 20 kA and 100 A, two novel techniques are developed, i.e., 1) the current-ratio transfer standard (CRTS), and 2) the improved equivalent ampere-turn method. The CRTS is a two-stage current transformer with two selectable current ratios of 500A/5A and 100A/5A. The two ratios are realized by the selection of parallel or series combination in the primary windings of the CRTS. The design and the performance of the 100-A primary standard, a coaxial-type self-calibratable current comparator, is also described.