Systematic error of time-division wattmeters when voltage and/or current are distorted

The mathematical models of three different electronic wattmeters based on time-division principles are derived. By using these models the systematic errors of such electronic wattmeters can be calculated for both sinusoidal and distorted voltage and/or current waveforms. It is emphasized that the systematic errors are a function of the ratio of the input and the internal converter frequency. The behavior of several wattmeters is analyzed theoretically and experimentally for a few practical situations. The conclusion is reached that time-division wattmeters can be used for high-precision power measurement when voltage and current are not distorted and also, with certain precautions, when they are distorted.