Digital Wattmeters for Loss Measurements of Power Transformers

This paper gives particular attention to the theoretical and practical difficulties that arise in the measurement of power losses in very large transformers or inductors. The most serious of these problems is the measurement of very low power factor loads, values down to 0.01 or 1% may be encountered. Major difficulties in making these measurements include problems associated with non-ideal power sources, problems introduced by the necessity - in some cases - of making measurements on highly distorted waveforms, and problems associated with making measurements in a restricted amount of time. While a measurement involves a complete system, the emphasis in this paper is upon the power measurement circuitry itself. An attempt is made to point out the theoretical limitations of various approaches and to separate these from the practical limitations of certain types of circuits. The major emphasis is upon digital methods of measuring power. The final section of the paper provides a comparison between some of the available types of digital wattmeters. It is hoped that this paper will provide help in selecting and in understanding the use of instruments to be used in the measurement of transformer losses.