An AC Current Transformer Standard Measuring System for Power Frequencies

This paper describes the design and setup of the new ac current transformer (CT) calibration system at Physikalisch-Technische Bundesanstalt, which uses enhanced, actively compensated current comparators (CCs) for rated primary currents from 5 A up to 60 kA, and a new CT bridge based on the differential method. The bridge makes use of modern, high-resolution analogto-digital converters, and makes it possible to connect traditional CTs as well as two-stage CTs with rated secondary currents from 100 mA to 5 A. The lower current ranges are integrated to enable a calibration of CTs of the highest available precision for national power standards. The current transducers and the two-channel sampling system of the bridge allow a resolution of approximately 10(-9), while the estimated Type B uncertainty of the bridge is around 10(-8).