The Application of Current Comparators in Instrumentation for Loss Measurements

A current comparator technique, applied to several auxiliary instruments, enables accurate power measurements to be made for measuring losses in medium and large transformers. The instruments include high voltage active dividers with nominal outputs of 120 volts, a current transformer with nominal output of 1A and precision wattmeters. Two stage compensated current transformer technology is used in the current transformers of each instrument to achieve uncertainties of < 10 PPM in magnitude and phase respectively. In the high voltage divider, the current comparator is used in a feedback loop to correct the magnitude and phase of the associated outputs. This paper discusses the technology used and the associated uncertainties of the instruments to achieve a system uncertainty of <50-PPM.