The Application of Current Comparators to the Calibration of Current Transformers at Ratios up to 36000/5 Amperes

The design and construction of a multiratio, 36 000-ampex e compensated current comparator and the technique by which it may be cascaded with a second multiratio comparator to calibrate, with a single balance, power frequency current transformers at ratios up to 7200/1 are described. The first comparator has a distributed, single-turn primary winding and a sectionalized 1200-turn secondary winding that may be series-paralleled to obtain eight ratios from 120/1 to 1200/1. A 300-kVA 1100-volt transformer is included in the single primary turn to provide the power required for the calibrations. In size the device is 40 inches in diameter and 20 inches in height, and its weight is 2700 lb. The two comparators are cascaded at nominal currents up to 30 amperes by connecting the secondary winding of the first comparator to the primary winding of the second and by making corresponding interconnections between the two compensation windings. At balance, the actual current ratio of the standard composed of the two comparators in cascade, or the first comparator alone, differs from the corresponding turns ratio by no more than a part per million (ppm).