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 of this study was to develop an instrument having measurement uncertainty of less than ±50 p.p.m. over these frequencies. The wattmeter was built around a wideband instrument developed earlier, and it uses 16-b analog-to-digital (A/D) converters and includes a two-stage current transformer in one of the input modules. It operates with asynchronous sampling, and high accuracy is achieved by approximately synchronizing the interval over which sampls 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 p.p.m. of full scale over the 50 Hz to 1000 Hz frequency range at all power factors