A Composite Current Transformer with Self-Calibration Uncertainty of 1 Ã\x97 10-9

This paper describes an absolute calibration method and the associated experimental device. A current ratio device with ratios from 1/N to (N + 1)/N is fabricated from (N + 1) current transformer units. The secondary current of each unit is 1/N of the primary current. The later is taken as reference. This composite current transformer can be self-calibrated by comparing the primary current in turn with every N out of (N + 1) currents of the secondary windings. The experimental device is a decade composite current transformer with relative accuracy of 2 × 10-7 at 2500 Hz. When using as a ratio standard, its ratio can be conveniently self-calibrated with an uncertainty of 1 in 109 (refer to input). Moreover, method of diminishing the capacitive errors of this current transformer is recommended. As a result, the operating frequency range of the composite transformer can be from 50 Hz to 15 000 Hz.