High-accuracy sampling wattmeter

A high-accuracy sampling wattmeter was developed to investigate the feasibility of using waveform sampling techniques for making very accurate power measurements at frequencies from 50 Hz to 1000 Hz. The goal was to build an instrument with an uncertainty of less than ±50 ppm over these frequencies. The new wattmeter uses 16-b converters and includes a two-stage current transformer in one of the modules. This wattmeter operates with asynchronous sampling. High-accuracy is achieved by approximately synchronizing the interval over which samples are taken with the period of the input signal. Special care was taken to design input stages with a flat frequency response and low temperature sensitivity. The wattmeter has been calibrated using an audio-frequency power bridge. The two instruments agreed to better than ±50 ppm of full scale over the 50 Hz to 1000 Hz frequency range at all power factors. The use of an RC attenuator for the voltage scaling proved to be marginal in frequency response for the 240 V range