HIGH-ACCURACY SAMPLING WATTMETER

A high-accuracy sampling wattmeter was deveoped at NIST (National Institute of Standards and Technology) to investigate the feasibility of using waveform sampling techniques for making very accurate power measurements at frequencies from 50 Hz to 1000 Hz. The prototype instrument was used to demonstrate the accuracy achievable with the sampling method. The goal of this study was to develop an instrument having measurement uncertainty of less than +/-50 ppm over these frequencies. The new high-accuracy sampling wattmeter was built around a wideband instrument developed earlier at NIST. The new wattmeter uses 16-bit analog-to-digital (A/D) converters and includes a two-stage current transformer in one of the input modules. This wattmeter, as the previous wattmeter, operates with asynchronous sampling. The 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 the NIST 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.