Wide-Band Two-Stage Current Transformers of High Accuracy

The design and performance of two high-accuracy transformers, identified as amplifier-aided two-stage transformers, are described. Each operates from 50 Hz to 10 kHz, supporting burdens up to 1 Ω. Self-contained ratios span from 5/5 to 100/5, with a rated secondary current of 5 A. Results indicate the errors at 10 kHz are within 30 and 15 ppm for the respective transformers, decreasing to less than 1 ppm at 1 kHz. Analysis of high-and low-frequency errors is presented, as well as a detailed description of a capacitance trimming technique effecting a factor-of-ten reduction of capacitive errors. Also described is a relatively simple step-up calibration system for determining the transformer errors. Calibration results are included.